draft-ietf-idr-bgp4-03.txt   draft-ietf-idr-bgp4-04.txt 
Network Working Group Y. Rekhter Network Working Group Y. Rekhter
INTERNET DRAFT cisco Systems INTERNET DRAFT cisco Systems
T.Li T.Li
Juniper Juniper Networks
Editors Editors
<draft-ietf-idr-bgp4-03.txt> August 1996 <draft-ietf-idr-bgp4-04.txt> October 1996
A Border Gateway Protocol 4 (BGP-4) A Border Gateway Protocol 4 (BGP-4)
Status of this Memo Status of this Memo
This document, together with its companion document, "Application of This document, together with its companion document, "Application of
the Border Gateway Protocol in the Internet", define an inter- the Border Gateway Protocol in the Internet", define an inter-
autonomous system routing protocol for the Internet. This document autonomous system routing protocol for the Internet. This document
specifies an IAB standards track protocol for the Internet community, specifies an IAB standards track protocol for the Internet community,
and requests discussion and suggestions for improvements. Please and requests discussion and suggestions for improvements. Please
refer to the current edition of the "IAB Official Protocol Standards" refer to the current edition of the "IAB Official Protocol Standards"
for the standardization state and status of this protocol. for the standardization state and status of this protocol. Distribu-
Distribution of this document is unlimited. tion of this document is unlimited.
This document is an Internet Draft. Internet Drafts are working This document is an Internet Draft. Internet Drafts are working docu-
documents of the Internet Engineering Task Force (IETF), its Areas, ments of the Internet Engineering Task Force (IETF), its Areas, and
and its Working Groups. Note that other groups may also distribute its Working Groups. Note that other groups may also distribute work-
working documents as Internet Drafts. ing documents as Internet Drafts.
Internet Drafts are draft documents valid for a maximum of six Internet Drafts are draft documents valid for a maximum of six
months. Internet Drafts may be updated, replaced, or obsoleted by months. Internet Drafts may be updated, replaced, or obsoleted by
other documents at any time. It is not appropriate to use Internet other documents at any time. It is not appropriate to use Internet
Drafts as reference material or to cite them other than as a "working Drafts as reference material or to cite them other than as a "working
draft" or "work in progress". draft" or "work in progress".
1. Acknowledgements 1. Acknowledgments
This document was originally published as RFC 1267 in October 1991, This document was originally published as RFC 1267 in October 1991,
jointly authored by Kirk Lougheed and Yakov Rekhter. jointly authored by Kirk Lougheed and Yakov Rekhter.
We would like to express our thanks to Guy Almes, Len Bosack, and We would like to express our thanks to Guy Almes, Len Bosack, and
Jeffrey C. Honig for their contributions to the earlier version of Jeffrey C. Honig for their contributions to the earlier version of
this document. this document.
We like to explicitly thank Bob Braden for the review of the earlier We like to explicitly thank Bob Braden for the review of the earlier
version of this document as well as his constructive and valuable version of this document as well as his constructive and valuable
comments. comments.
We would also like to thank Bob Hinden, Director for Routing of the We would also like to thank Bob Hinden, Director for Routing of the
Internet Engineering Steering Group, and the team of reviewers he Internet Engineering Steering Group, and the team of reviewers he
assembled to review the previous version (BGP-2) of this document. assembled to review the previous version (BGP-2) of this document.
This team, consisting of Deborah Estrin, Milo Medin, John Moy, Radia This team, consisting of Deborah Estrin, Milo Medin, John Moy, Radia
Perlman, Martha Steenstrup, Mike St. Johns, and Paul Tsuchiya, acted Perlman, Martha Steenstrup, Mike St. Johns, and Paul Tsuchiya, acted
with a strong combination of toughness, professionalism, and with a strong combination of toughness, professionalism, and cour-
courtesy. tesy.
This updated version of the document is the product of the IETF IDR This updated version of the document is the product of the IETF IDR
Working Group with Yakov Rekhter and Tony Li as editors. Certain Working Group with Yakov Rekhter and Tony Li as editors. Certain sec-
sections of the document borrowed heavily from IDRP [7], which is the tions of the document borrowed heavily from IDRP [7], which is the
OSI counterpart of BGP. For this credit should be given to the ANSI OSI counterpart of BGP. For this credit should be given to the ANSI
X3S3.3 group chaired by Lyman Chapin and to Charles Kunzinger who was X3S3.3 group chaired by Lyman Chapin and to Charles Kunzinger who was
the IDRP editor within that group. We would also like to thank Mike the IDRP editor within that group. We would also like to thank Mike
Craren, Dimitry Haskin, John Krawczyk, and Paul Traina for their Craren, Dimitry Haskin, John Krawczyk, David LeRoy, John Scudder,
insightful comments. Paul Traina, and Curtis Villamizar for their comments.
We would like to specially acknowledge numerous contributions by We would like to specially acknowledge numerous contributions by Den-
Dennis Ferguson. nis Ferguson.
2. Introduction 2. Introduction
The Border Gateway Protocol (BGP) is an inter-Autonomous System The Border Gateway Protocol (BGP) is an inter-Autonomous System rout-
routing protocol. It is built on experience gained with EGP as ing protocol. It is built on experience gained with EGP as defined
defined in RFC 904 [1] and EGP usage in the NSFNET Backbone as in RFC 904 [1] and EGP usage in the NSFNET Backbone as described in
described in RFC 1092 [2] and RFC 1093 [3]. RFC 1092 [2] and RFC 1093 [3].
The primary function of a BGP speaking system is to exchange network The primary function of a BGP speaking system is to exchange network
reachability information with other BGP systems. This network reachability information with other BGP systems. This network reach-
reachability information includes information on the list of ability information includes information on the list of Autonomous
Autonomous Systems (ASs) that reachability information traverses. Systems (ASs) that reachability information traverses. This informa-
This information is sufficient to construct a graph of AS tion is sufficient to construct a graph of AS connectivity from which
connectivity from which routing loops may be pruned and some policy routing loops may be pruned and some policy decisions at the AS level
decisions at the AS level may be enforced. may be enforced.
BGP-4 provides a new set of mechanisms for supporting classless BGP-4 provides a new set of mechanisms for supporting classless
interdomain routing. These mechanisms include support for interdomain routing. These mechanisms include support for advertis-
advertising an IP prefix and eliminates the concept of network ing an IP prefix and eliminates the concept of network "class" within
"class" within BGP. BGP-4 also introduces mechanisms which allow BGP. BGP-4 also introduces mechanisms which allow aggregation of
aggregation of routes, including aggregation of AS paths. These routes, including aggregation of AS paths. These changes provide
changes provide support for the proposed supernetting scheme [8, 9]. support for the proposed supernetting scheme [8, 9].
To characterize the set of policy decisions that can be enforced To characterize the set of policy decisions that can be enforced
using BGP, one must focus on the rule that a BGP speaker advertise to using BGP, one must focus on the rule that a BGP speaker advertise to
its peers (other BGP speakers which it communicates with) in its peers (other BGP speakers which it communicates with) in neigh-
neighboring ASs only those routes that it itself uses. This rule boring ASs only those routes that it itself uses. This rule reflects
reflects the "hop-by-hop" routing paradigm generally used throughout the "hop-by-hop" routing paradigm generally used throughout the
the current Internet. Note that some policies cannot be supported by current Internet. Note that some policies cannot be supported by the
the "hop-by-hop" routing paradigm and thus require techniques such as "hop-by-hop" routing paradigm and thus require techniques such as
source routing to enforce. For example, BGP does not enable one AS source routing to enforce. For example, BGP does not enable one AS
to send traffic to a neighboring AS intending that the traffic take a to send traffic to a neighboring AS intending that the traffic take a
different route from that taken by traffic originating in the different route from that taken by traffic originating in the neigh-
neighboring AS. On the other hand, BGP can support any policy boring AS. On the other hand, BGP can support any policy conforming
conforming to the "hop-by-hop" routing paradigm. Since the current to the "hop-by-hop" routing paradigm. Since the current Internet
Internet uses only the "hop-by-hop" routing paradigm and since BGP uses only the "hop-by-hop" routing paradigm and since BGP can support
can support any policy that conforms to that paradigm, BGP is highly any policy that conforms to that paradigm, BGP is highly applicable
applicable as an inter-AS routing protocol for the current Internet. as an inter-AS routing protocol for the current Internet.
A more complete discussion of what policies can and cannot be A more complete discussion of what policies can and cannot be
enforced with BGP is outside the scope of this document (but refer to enforced with BGP is outside the scope of this document (but refer to
the companion document discussing BGP usage [5]). the companion document discussing BGP usage [5]).
BGP runs over a reliable transport protocol. This eliminates the BGP runs over a reliable transport protocol. This eliminates the
need to implement explicit update fragmentation, retransmission, need to implement explicit update fragmentation, retransmission,
acknowledgement, and sequencing. Any authentication scheme used by acknowledgment, and sequencing. Any authentication scheme used by
the transport protocol may be used in addition to BGP's own the transport protocol may be used in addition to BGP's own authenti-
authentication mechanisms. The error notification mechanism used in cation mechanisms. The error notification mechanism used in BGP
BGP assumes that the transport protocol supports a "graceful" close, assumes that the transport protocol supports a "graceful" close,
i.e., that all outstanding data will be delivered before the i.e., that all outstanding data will be delivered before the connec-
connection is closed. tion is closed.
BGP uses TCP [4] as its transport protocol. TCP meets BGP's BGP uses TCP [4] as its transport protocol. TCP meets BGP's trans-
transport requirements and is present in virtually all commercial port requirements and is present in virtually all commercial routers
routers and hosts. In the following descriptions the phrase and hosts. In the following descriptions the phrase "transport pro-
"transport protocol connection" can be understood to refer to a TCP tocol connection" can be understood to refer to a TCP connection.
connection. BGP uses TCP port 179 for establishing its connections. BGP uses TCP port 179 for establishing its connections.
This document uses the term `Autonomous System' (AS) throughout. The This document uses the term `Autonomous System' (AS) throughout. The
classic definition of an Autonomous System is a set of routers under classic definition of an Autonomous System is a set of routers under
a single technical administration, using an interior gateway protocol a single technical administration, using an interior gateway protocol
and common metrics to route packets within the AS, and using an and common metrics to route packets within the AS, and using an exte-
exterior gateway protocol to route packets to other ASs. Since this rior gateway protocol to route packets to other ASs. Since this
classic definition was developed, it has become common for a single classic definition was developed, it has become common for a single
AS to use several interior gateway protocols and sometimes several AS to use several interior gateway protocols and sometimes several
sets of metrics within an AS. The use of the term Autonomous System sets of metrics within an AS. The use of the term Autonomous System
here stresses the fact that, even when multiple IGPs and metrics are here stresses the fact that, even when multiple IGPs and metrics are
used, the administration of an AS appears to other ASs to have a used, the administration of an AS appears to other ASs to have a sin-
single coherent interior routing plan and presents a consistent gle coherent interior routing plan and presents a consistent picture
picture of what destinations are reachable through it. of what destinations are reachable through it.
The planned use of BGP in the Internet environment, including such The planned use of BGP in the Internet environment, including such
issues as topology, the interaction between BGP and IGPs, and the issues as topology, the interaction between BGP and IGPs, and the
enforcement of routing policy rules is presented in a companion enforcement of routing policy rules is presented in a companion docu-
document [5]. This document is the first of a series of documents ment [5]. This document is the first of a series of documents
planned to explore various aspects of BGP application. Please send planned to explore various aspects of BGP application. Please send
comments to the BGP mailing list (bgp@ans.net). comments to the BGP mailing list (bgp@ans.net).
3. Summary of Operation 3. Summary of Operation
Two systems form a transport protocol connection between one another. Two systems form a transport protocol connection between one another.
They exchange messages to open and confirm the connection parameters. They exchange messages to open and confirm the connection parameters.
The initial data flow is the entire BGP routing table. Incremental The initial data flow is the entire BGP routing table. Incremental
updates are sent as the routing tables change. BGP does not require updates are sent as the routing tables change. BGP does not require
periodic refresh of the entire BGP routing table. Therefore, a BGP periodic refresh of the entire BGP routing table. Therefore, a BGP
speaker must retain the current version of the entire BGP routing speaker must retain the current version of the entire BGP routing
tables of all of its peers for the duration of the connection. tables of all of its peers for the duration of the connection.
KeepAlive messages are sent periodically to ensure the liveness of KeepAlive messages are sent periodically to ensure the liveness of
the connection. Notification messages are sent in response to errors the connection. Notification messages are sent in response to errors
or special conditions. If a connection encounters an error or special conditions. If a connection encounters an error condi-
condition, a notification message is sent and the connection is tion, a notification message is sent and the connection is closed.
closed.
The hosts executing the Border Gateway Protocol need not be routers. The hosts executing the Border Gateway Protocol need not be routers.
A non-routing host could exchange routing information with routers A non-routing host could exchange routing information with routers
via EGP or even an interior routing protocol. That non-routing host via EGP or even an interior routing protocol. That non-routing host
could then use BGP to exchange routing information with a border could then use BGP to exchange routing information with a border
router in another Autonomous System. The implications and router in another Autonomous System. The implications and applica-
applications of this architecture are for further study. tions of this architecture are for further study.
Connections between BGP speakers of different ASs are referred to as
"external" links. BGP connections between BGP speakers within the
same AS are referred to as "internal" links. Similarly, a peer in a
different AS is referred to as an external peer, while a peer in the
same AS may be described as an internal peer. Internal BGP and
external BGP are commonly abbreviated IBGP and EBGP.
If a particular AS has multiple BGP speakers and is providing transit If a particular AS has multiple BGP speakers and is providing transit
service for other ASs, then care must be taken to ensure a consistent service for other ASs, then care must be taken to ensure a consistent
view of routing within the AS. A consistent view of the interior view of routing within the AS. A consistent view of the interior
routes of the AS is provided by the interior routing protocol. A routes of the AS is provided by the interior routing protocol. A
consistent view of the routes exterior to the AS can be provided by consistent view of the routes exterior to the AS can be provided by
having all BGP speakers within the AS maintain direct BGP connections having all BGP speakers within the AS maintain direct IBGP connec-
with each other. Using a common set of policies, the BGP speakers tions with each other. Alternately the interior routing protocol can
arrive at an agreement as to which border routers will serve as pass BGP information among routers within an AS, taking care not to
exit/entry points for particular destinations outside the AS. This lose BGP attributes that will be needed by EBGP speakers if transit
information is communicated to the AS's internal routers, possibly connectivity is being provided. For the purpose of discussion, it is
via the interior routing protocol. Care must be taken to ensure that assumed that BGP information is passed within an AS using IBGP. Care
the interior routers have all been updated with transit information must be taken to ensure that the interior routers have all been
before the BGP speakers announce to other ASs that transit service is updated with transit information before the EBGP speakers announce to
being provided. other ASs that transit service is being provided.
Connections between BGP speakers of different ASs are referred to as
"external" links. BGP connections between BGP speakers within the
same AS are referred to as "internal" links. Similarly, a peer in a
different AS is referred to as an external peer, while a peer in the
same AS may be described as an internal peer.
3.1 Routes: Advertisement and Storage 3.1 Routes: Advertisement and Storage
For purposes of this protocol a route is defined as a unit of For purposes of this protocol a route is defined as a unit of infor-
information that pairs a destination with the attributes of a path to mation that pairs a destination with the attributes of a path to that
that destination: destination:
- Routes are advertised between a pair of BGP speakers in UPDATE - Routes are advertised between a pair of BGP speakers in UPDATE
messages: the destination is the systems whose IP addresses are messages: the destination is the systems whose IP addresses are
reported in the Network Layer Reachability Information (NLRI) reported in the Network Layer Reachability Information (NLRI)
field, and the the path is the information reported in the path field, and the the path is the information reported in the path
attributes fields of the same UPDATE message. attributes fields of the same UPDATE message.
- Routes are stored in the Routing Information Bases (RIBs): - Routes are stored in the Routing Information Bases (RIBs):
namely, the Adj-RIBs-In, the Loc-RIB, and the Adj-RIBs-Out. Routes namely, the Adj-RIBs-In, the Loc-RIB, and the Adj-RIBs-Out. Routes
that will be advertised to other BGP speakers must be present in that will be advertised to other BGP speakers must be present in
skipping to change at page 6, line 11 skipping to change at page 6, line 11
c) the BGP speaker - BGP speaker connection can be closed, which c) the BGP speaker - BGP speaker connection can be closed, which
implicitly removes from service all routes which the pair of implicitly removes from service all routes which the pair of
speakers had advertised to each other. speakers had advertised to each other.
3.2 Routing Information Bases 3.2 Routing Information Bases
The Routing Information Base (RIB) within a BGP speaker consists of The Routing Information Base (RIB) within a BGP speaker consists of
three distinct parts: three distinct parts:
a) Adj-RIBs-In: The Adj-RIBs-In store routing information that has a) Adj-RIBs-In: The Adj-RIBs-In store routing information that has
been learned from inbound UPDATE messages. Their contents been learned from inbound UPDATE messages. Their contents repre-
represent routes that are available as an input to the Decision sent routes that are available as an input to the Decision Pro-
Process. cess.
b) Loc-RIB: The Loc-RIB contains the local routing information b) Loc-RIB: The Loc-RIB contains the local routing information
that the BGP speaker has selected by applying its local policies that the BGP speaker has selected by applying its local policies
to the routing information contained in its Adj-RIBs-In. to the routing information contained in its Adj-RIBs-In.
c) Adj-RIBs-Out: The Adj-RIBs-Out store the information that the c) Adj-RIBs-Out: The Adj-RIBs-Out store the information that the
local BGP speaker has selected for advertisement to its peers. The local BGP speaker has selected for advertisement to its peers. The
routing information stored in the Adj-RIBs-Out will be carried in routing information stored in the Adj-RIBs-Out will be carried in
the local BGP speaker's UPDATE messages and advertised to its the local BGP speaker's UPDATE messages and advertised to its
peers. peers.
In summary, the Adj-RIBs-In contain unprocessed routing information In summary, the Adj-RIBs-In contain unprocessed routing information
that has been advertised to the local BGP speaker by its peers; the that has been advertised to the local BGP speaker by its peers; the
Loc-RIB contains the routes that have been selected by the local BGP Loc-RIB contains the routes that have been selected by the local BGP
speaker's Decision Process; and the Adj-RIBs-Out organize the routes speaker's Decision Process; and the Adj-RIBs-Out organize the routes
for advertisement to specific peers by means of the local speaker's for advertisement to specific peers by means of the local speaker's
UPDATE messages. UPDATE messages.
Although the conceptual model distinguishes between Adj-RIBs-In, Although the conceptual model distinguishes between Adj-RIBs-In, Loc-
Loc-RIB, and Adj-RIBs-Out, this neither implies nor requires that an RIB, and Adj-RIBs-Out, this neither implies nor requires that an
implementation must maintain three separate copies of the routing implementation must maintain three separate copies of the routing
information. The choice of implementation (for example, 3 copies of information. The choice of implementation (for example, 3 copies of
the information vs 1 copy with pointers) is not constrained by the the information vs 1 copy with pointers) is not constrained by the
protocol. protocol.
4. Message Formats 4. Message Formats
This section describes message formats used by BGP. This section describes message formats used by BGP.
Messages are sent over a reliable transport protocol connection. A Messages are sent over a reliable transport protocol connection. A
skipping to change at page 7, line 30 skipping to change at page 7, line 30
| | | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Length | Type | | Length | Type |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Marker: Marker:
This 16-octet field contains a value that the receiver of the This 16-octet field contains a value that the receiver of the
message can predict. If the Type of the message is OPEN, or if message can predict. If the Type of the message is OPEN, or if
the OPEN message carries no Authentication Information (as an the OPEN message carries no Authentication Information (as an
Optional Parameter), then the Marker must be all ones. Optional Parameter), then the Marker must be all ones. Other-
Otherwise, the value of the marker can be predicted by some a wise, the value of the marker can be predicted by some a compu-
computation specified as part of the authentication mechanism tation specified as part of the authentication mechanism (which
(which is specified as part of the Authentication Information) is specified as part of the Authentication Information) used.
used. The Marker can be used to detect loss of synchronization The Marker can be used to detect loss of synchronization
between a pair of BGP peers, and to authenticate incoming BGP between a pair of BGP peers, and to authenticate incoming BGP
messages. messages.
Length: Length:
This 2-octet unsigned integer indicates the total length of the This 2-octet unsigned integer indicates the total length of the
message, including the header, in octets. Thus, e.g., it message, including the header, in octets. Thus, e.g., it
allows one to locate in the transport-level stream the (Marker allows one to locate in the transport-level stream the (Marker
field of the) next message. The value of the Length field must field of the) next message. The value of the Length field must
always be at least 19 and no greater than 4096, and may be always be at least 19 and no greater than 4096, and may be fur-
further constrained, depending on the message type. No ther constrained, depending on the message type. No "padding"
"padding" of extra data after the message is allowed, so the of extra data after the message is allowed, so the Length field
Length field must have the smallest value required given the must have the smallest value required given the rest of the
rest of the message. message.
Type: Type:
This 1-octet unsigned integer indicates the type code of the This 1-octet unsigned integer indicates the type code of the
message. The following type codes are defined: message. The following type codes are defined:
1 - OPEN 1 - OPEN
2 - UPDATE 2 - UPDATE
3 - NOTIFICATION 3 - NOTIFICATION
4 - KEEPALIVE 4 - KEEPALIVE
4.2 OPEN Message Format 4.2 OPEN Message Format
After a transport protocol connection is established, the first After a transport protocol connection is established, the first mes-
message sent by each side is an OPEN message. If the OPEN message is sage sent by each side is an OPEN message. If the OPEN message is
acceptable, a KEEPALIVE message confirming the OPEN is sent back. acceptable, a KEEPALIVE message confirming the OPEN is sent back.
Once the OPEN is confirmed, UPDATE, KEEPALIVE, and NOTIFICATION Once the OPEN is confirmed, UPDATE, KEEPALIVE, and NOTIFICATION mes-
messages may be exchanged. sages may be exchanged.
In addition to the fixed-size BGP header, the OPEN message contains In addition to the fixed-size BGP header, the OPEN message contains
the following fields: the following fields:
0 1 2 3 0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+
| Version | | Version |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| My Autonomous System | | My Autonomous System |
skipping to change at page 9, line 31 skipping to change at page 9, line 31
value of the Hold Timer by using the smaller of its configured value of the Hold Timer by using the smaller of its configured
Hold Time and the Hold Time received in the OPEN message. The Hold Time and the Hold Time received in the OPEN message. The
Hold Time MUST be either zero or at least three seconds. An Hold Time MUST be either zero or at least three seconds. An
implementation may reject connections on the basis of the Hold implementation may reject connections on the basis of the Hold
Time. The calculated value indicates the maximum number of Time. The calculated value indicates the maximum number of
seconds that may elapse between the receipt of successive seconds that may elapse between the receipt of successive
KEEPALIVE, and/or UPDATE messages by the sender. KEEPALIVE, and/or UPDATE messages by the sender.
BGP Identifier: BGP Identifier:
This 4-octet unsigned integer indicates the BGP Identifier of This 4-octet unsigned integer indicates the BGP Identifier of
the sender. A given BGP speaker sets the value of its BGP the sender. A given BGP speaker sets the value of its BGP Iden-
Identifier to an IP address assigned to that BGP speaker. The tifier to an IP address assigned to that BGP speaker. The
value of the BGP Identifier is determined on startup and is the value of the BGP Identifier is determined on startup and is the
same for every local interface and every BGP peer. same for every local interface and every BGP peer.
Optional Parameters Length: Optional Parameters Length:
This 1-octet unsigned integer indicates the total length of the This 1-octet unsigned integer indicates the total length of the
Optional Parameters field in octets. If the value of this field Optional Parameters field in octets. If the value of this field
is zero, no Optional Parameters are present. is zero, no Optional Parameters are present.
Optional Parameters: Optional Parameters:
skipping to change at page 10, line 11 skipping to change at page 10, line 11
This field may contain a list of optional parameters, where This field may contain a list of optional parameters, where
each parameter is encoded as a <Parameter Type, Parameter each parameter is encoded as a <Parameter Type, Parameter
Length, Parameter Value> triplet. Length, Parameter Value> triplet.
0 1 0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-...
| Parm. Type | Parm. Length | Parameter Value (variable) | Parm. Type | Parm. Length | Parameter Value (variable)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-...
Parameter Type is a one octet field that unambiguously Parameter Type is a one octet field that unambiguously identi-
identifies individual parameters. Parameter Length is a one fies individual parameters. Parameter Length is a one octet
octet field that contains the length of the Parameter Value field that contains the length of the Parameter Value field in
field in octets. Parameter Value is a variable length field octets. Parameter Value is a variable length field that is
that is interpreted according to the value of the Parameter interpreted according to the value of the Parameter Type field.
Type field.
This document defines the following Optional Parameters: This document defines the following Optional Parameters:
a) Authentication Information (Parameter Type 1): a) Authentication Information (Parameter Type 1):
This optional parameter may be used to authenticate a BGP This optional parameter may be used to authenticate a BGP
peer. The Parameter Value field contains a 1-octet peer. The Parameter Value field contains a 1-octet Authenti-
Authentication Code followed by a variable length cation Code followed by a variable length Authentication
Authentication Data. Data.
0 1 2 3 4 5 6 7 8 0 1 2 3 4 5 6 7 8
+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+
| Auth. Code | | Auth. Code |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| | | |
| Authentication Data | | Authentication Data |
| | | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Authentication Code: Authentication Code:
This 1-octet unsigned integer indicates the This 1-octet unsigned integer indicates the authenti-
authentication mechanism being used. Whenever an cation mechanism being used. Whenever an authentica-
authentication mechanism is specified for use within tion mechanism is specified for use within BGP, three
BGP, three things must be included in the things must be included in the specification:
specification:
- the value of the Authentication Code which indicates - the value of the Authentication Code which indicates
use of the mechanism, use of the mechanism,
- the form and meaning of the Authentication Data, and - the form and meaning of the Authentication Data, and
- the algorithm for computing values of Marker fields. - the algorithm for computing values of Marker fields.
Note that a separate authentication mechanism may be Note that a separate authentication mechanism may be
used in establishing the transport level connection. used in establishing the transport level connection.
Authentication Data: Authentication Data:
The form and meaning of this field is a variable- The form and meaning of this field is a variable-
length field depend on the Authentication Code. length field depend on the Authentication Code.
The minimum length of the OPEN message is 29 octets (including The minimum length of the OPEN message is 29 octets (including
message header). message header).
4.3 UPDATE Message Format 4.3 UPDATE Message Format
UPDATE messages are used to transfer routing information between BGP UPDATE messages are used to transfer routing information between BGP
peers. The information in the UPDATE packet can be used to construct peers. The information in the UPDATE packet can be used to construct
a graph describing the relationships of the various Autonomous a graph describing the relationships of the various Autonomous Sys-
Systems. By applying rules to be discussed, routing information tems. By applying rules to be discussed, routing information loops
loops and some other anomalies may be detected and removed from and some other anomalies may be detected and removed from inter-AS
inter-AS routing. routing.
An UPDATE message is used to advertise a single feasible route to a An UPDATE message is used to advertise a single feasible route to a
peer, or to withdraw multiple unfeasible routes from service (see peer, or to withdraw multiple unfeasible routes from service (see
3.1). An UPDATE message may simultaneously advertise a feasible route 3.1). An UPDATE message may simultaneously advertise a feasible route
and withdraw multiple unfeasible routes from service. The UPDATE and withdraw multiple unfeasible routes from service. The UPDATE
message always includes the fixed-size BGP header, and can optionally message always includes the fixed-size BGP header, and can optionally
include the other fields as shown below: include the other fields as shown below:
+-----------------------------------------------------+ +-----------------------------------------------------+
| Unfeasible Routes Length (2 octets) | | Unfeasible Routes Length (2 octets) |
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set to 1) or well-known (if set to 0). set to 1) or well-known (if set to 0).
The second high-order bit (bit 1) of the Attribute Flags octet The second high-order bit (bit 1) of the Attribute Flags octet
is the Transitive bit. It defines whether an optional is the Transitive bit. It defines whether an optional
attribute is transitive (if set to 1) or non-transitive (if set attribute is transitive (if set to 1) or non-transitive (if set
to 0). For well-known attributes, the Transitive bit must be to 0). For well-known attributes, the Transitive bit must be
set to 1. (See Section 5 for a discussion of transitive set to 1. (See Section 5 for a discussion of transitive
attributes.) attributes.)
The third high-order bit (bit 2) of the Attribute Flags octet The third high-order bit (bit 2) of the Attribute Flags octet
is the Partial bit. It defines whether the information is the Partial bit. It defines whether the information con-
contained in the optional transitive attribute is partial (if tained in the optional transitive attribute is partial (if set
set to 1) or complete (if set to 0). For well-known attributes to 1) or complete (if set to 0). For well-known attributes and
and for optional non-transitive attributes the Partial bit must for optional non-transitive attributes the Partial bit must be
be set to 0. set to 0.
The fourth high-order bit (bit 3) of the Attribute Flags octet The fourth high-order bit (bit 3) of the Attribute Flags octet
is the Extended Length bit. It defines whether the Attribute is the Extended Length bit. It defines whether the Attribute
Length is one octet (if set to 0) or two octets (if set to 1). Length is one octet (if set to 0) or two octets (if set to 1).
Extended Length may be used only if the length of the attribute Extended Length may be used only if the length of the attribute
value is greater than 255 octets. value is greater than 255 octets.
The lower-order four bits of the Attribute Flags octet are . The lower-order four bits of the Attribute Flags octet are .
unused. They must be zero (and must be ignored when received). unused. They must be zero (and must be ignored when received).
skipping to change at page 14, line 47 skipping to change at page 14, line 44
Its usage is defined in 5.1.1 Its usage is defined in 5.1.1
b) AS_PATH (Type Code 2): b) AS_PATH (Type Code 2):
AS_PATH is a well-known mandatory attribute that is composed AS_PATH is a well-known mandatory attribute that is composed
of a sequence of AS path segments. Each AS path segment is of a sequence of AS path segments. Each AS path segment is
represented by a triple <path segment type, path segment represented by a triple <path segment type, path segment
length, path segment value>. length, path segment value>.
The path segment type is a 1-octet long field with the The path segment type is a 1-octet long field with the fol-
following values defined: lowing values defined:
Value Segment Type Value Segment Type
1 AS_SET: unordered set of ASs a route in the 1 AS_SET: unordered set of ASs a route in the
UPDATE message has traversed UPDATE message has traversed
2 AS_SEQUENCE: ordered set of ASs a route in 2 AS_SEQUENCE: ordered set of ASs a route in
the UPDATE message has traversed the UPDATE message has traversed
The path segment length is a 1-octet long field containing The path segment length is a 1-octet long field containing
the number of ASs in the path segment value field. the number of ASs in the path segment value field.
The path segment value field contains one or more AS The path segment value field contains one or more AS num-
numbers, each encoded as a 2-octets long field. bers, each encoded as a 2-octets long field.
Usage of this attribute is defined in 5.1.2. Usage of this attribute is defined in 5.1.2.
c) NEXT_HOP (Type Code 3): c) NEXT_HOP (Type Code 3):
This is a well-known mandatory attribute that defines the IP This is a well-known mandatory attribute that defines the IP
address of the border router that should be used as the next address of the border router that should be used as the next
hop to the destinations listed in the Network Layer hop to the destinations listed in the Network Layer Reacha-
Reachability field of the UPDATE message. bility field of the UPDATE message.
Usage of this attribute is defined in 5.1.3. Usage of this attribute is defined in 5.1.3.
d) MULTI_EXIT_DISC (Type Code 4): d) MULTI_EXIT_DISC (Type Code 4):
This is an optional non-transitive attribute that is a four This is an optional non-transitive attribute that is a four
octet non-negative integer. The value of this attribute may octet non-negative integer. The value of this attribute may
be used by a BGP speaker's decision process to discriminate be used by a BGP speaker's decision process to discriminate
among multiple exit points to a neighboring autonomous among multiple exit points to a neighboring autonomous sys-
system. tem.
Its usage is defined in 5.1.4. Its usage is defined in 5.1.4.
e) LOCAL_PREF (Type Code 5): e) LOCAL_PREF (Type Code 5):
LOCAL_PREF is a well-known discretionary attribute that is a LOCAL_PREF is a well-known mandatory attribute that is a
four octet non-negative integer. It is used by a BGP speaker four octet non-negative integer. It is used by a BGP speaker
to inform other BGP speakers in its own autonomous system of to inform other internal peers of the originating speaker's
the originating speaker's degree of preference for an degree of preference for an advertised route. Usage of this
advertised route. Usage of this attribute is described in attribute is described in 5.1.5.
5.1.5.
f) ATOMIC_AGGREGATE (Type Code 6) f) ATOMIC_AGGREGATE (Type Code 6)
ATOMIC_AGGREGATE is a well-known discretionary attribute of ATOMIC_AGGREGATE is a well-known discretionary attribute of
length 0. It is used by a BGP speaker to inform other BGP length 0. It is used by a BGP speaker to inform other BGP
speakers that the local system selected a less specific speakers that the local system selected a less specific
route without selecting a more specific route which is route without selecting a more specific route which is
included in it. Usage of this attribute is described in included in it. Usage of this attribute is described in
5.1.6. 5.1.6.
skipping to change at page 16, line 8 skipping to change at page 16, line 4
f) ATOMIC_AGGREGATE (Type Code 6) f) ATOMIC_AGGREGATE (Type Code 6)
ATOMIC_AGGREGATE is a well-known discretionary attribute of ATOMIC_AGGREGATE is a well-known discretionary attribute of
length 0. It is used by a BGP speaker to inform other BGP length 0. It is used by a BGP speaker to inform other BGP
speakers that the local system selected a less specific speakers that the local system selected a less specific
route without selecting a more specific route which is route without selecting a more specific route which is
included in it. Usage of this attribute is described in included in it. Usage of this attribute is described in
5.1.6. 5.1.6.
g) AGGREGATOR (Type Code 7) g) AGGREGATOR (Type Code 7)
AGGREGATOR is an optional transitive attribute of length 6. AGGREGATOR is an optional transitive attribute of length 6.
The attribute contains the last AS number that formed the The attribute contains the last AS number that formed the
aggregate route (encoded as 2 octets), followed by the IP aggregate route (encoded as 2 octets), followed by the IP
address of the BGP speaker that formed the aggregate route address of the BGP speaker that formed the aggregate route
(encoded as 4 octets). Usage of this attribute is described (encoded as 4 octets). Usage of this attribute is described
in 5.1.7 in 5.1.7
Network Layer Reachability Information: Network Layer Reachability Information:
This variable length field contains a list of IP address This variable length field contains a list of IP address pre-
prefixes. The length in octets of the Network Layer fixes. The length in octets of the Network Layer Reachability
Reachability Information is not encoded explicitly, but can be Information is not encoded explicitly, but can be calculated
calculated as: as:
UPDATE message Length - 23 - Total Path Attributes Length - UPDATE message Length - 23 - Total Path Attributes Length -
Unfeasible Routes Length Unfeasible Routes Length
where UPDATE message Length is the value encoded in the fixed- where UPDATE message Length is the value encoded in the fixed-
size BGP header, Total Path Attribute Length and Unfeasible size BGP header, Total Path Attribute Length and Unfeasible
Routes Length are the values encoded in the variable part of Routes Length are the values encoded in the variable part of
the UPDATE message, and 23 is a combined length of the fixed- the UPDATE message, and 23 is a combined length of the fixed-
size BGP header, the Total Path Attribute Length field and the size BGP header, the Total Path Attribute Length field and the
Unfeasible Routes Length field. Unfeasible Routes Length field.
skipping to change at page 17, line 23 skipping to change at page 17, line 18
for the fixed header + 2 octets for the Unfeasible Routes Length + 2 for the fixed header + 2 octets for the Unfeasible Routes Length + 2
octets for the Total Path Attribute Length (the value of Unfeasible octets for the Total Path Attribute Length (the value of Unfeasible
Routes Length is 0 and the value of Total Path Attribute Length is Routes Length is 0 and the value of Total Path Attribute Length is
0). 0).
An UPDATE message can advertise at most one route, which may be An UPDATE message can advertise at most one route, which may be
described by several path attributes. All path attributes contained described by several path attributes. All path attributes contained
in a given UPDATE messages apply to the destinations carried in the in a given UPDATE messages apply to the destinations carried in the
Network Layer Reachability Information field of the UPDATE message. Network Layer Reachability Information field of the UPDATE message.
An UPDATE message can list multiple routes to be withdrawn from An UPDATE message can list multiple routes to be withdrawn from ser-
service. Each such route is identified by its destination (expressed vice. Each such route is identified by its destination (expressed as
as an IP prefix), which unambiguously identifies the route in the an IP prefix), which unambiguously identifies the route in the con-
context of the BGP speaker - BGP speaker connection to which it has text of the BGP speaker - BGP speaker connection to which it has been
been previously been advertised. previously been advertised.
An UPDATE message may advertise only routes to be withdrawn from An UPDATE message may advertise only routes to be withdrawn from ser-
service, in which case it will not include path attributes or Network vice, in which case it will not include path attributes or Network
Layer Reachability Information. Conversely, it may advertise only a Layer Reachability Information. Conversely, it may advertise only a
feasible route, in which case the WITHDRAWN ROUTES field need not be feasible route, in which case the WITHDRAWN ROUTES field need not be
present. present.
4.4 KEEPALIVE Message Format 4.4 KEEPALIVE Message Format
BGP does not use any transport protocol-based keep-alive mechanism to BGP does not use any transport protocol-based keep-alive mechanism to
determine if peers are reachable. Instead, KEEPALIVE messages are determine if peers are reachable. Instead, KEEPALIVE messages are
exchanged between peers often enough as not to cause the Hold Timer exchanged between peers often enough as not to cause the Hold Timer
to expire. A reasonable maximum time between KEEPALIVE messages to expire. A reasonable maximum time between KEEPALIVE messages
would be one third of the Hold Time interval. KEEPALIVE messages would be one third of the Hold Time interval. KEEPALIVE messages
MUST NOT be sent more frequently than one per second. An MUST NOT be sent more frequently than one per second. An implementa-
implementation MAY adjust the rate at which it sends KEEPALIVE tion MAY adjust the rate at which it sends KEEPALIVE messages as a
messages as a function of the Hold Time interval. function of the Hold Time interval.
If the negotiated Hold Time interval is zero, then periodic KEEPALIVE If the negotiated Hold Time interval is zero, then periodic KEEPALIVE
messages MUST NOT be sent. messages MUST NOT be sent.
KEEPALIVE message consists of only message header and has a length of KEEPALIVE message consists of only message header and has a length of
19 octets. 19 octets.
4.5 NOTIFICATION Message Format 4.5 NOTIFICATION Message Format
A NOTIFICATION message is sent when an error condition is detected. A NOTIFICATION message is sent when an error condition is detected.
skipping to change at page 18, line 23 skipping to change at page 18, line 23
0 1 2 3 0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Error code | Error subcode | Data | | Error code | Error subcode | Data |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +
| | | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Error Code: Error Code:
This 1-octet unsigned integer indicates the type of This 1-octet unsigned integer indicates the type of NOTIFICA-
NOTIFICATION. The following Error Codes have been defined: TION. The following Error Codes have been defined:
Error Code Symbolic Name Reference Error Code Symbolic Name Reference
1 Message Header Error Section 6.1 1 Message Header Error Section 6.1
2 OPEN Message Error Section 6.2 2 OPEN Message Error Section 6.2
3 UPDATE Message Error Section 6.3 3 UPDATE Message Error Section 6.3
4 Hold Timer Expired Section 6.5 4 Hold Timer Expired Section 6.5
5 Finite State Machine Error Section 6.6 5 Finite State Machine Error Section 6.6
6 Cease Section 6.7 6 Cease Section 6.7
Error subcode: Error subcode:
This 1-octet unsigned integer provides more specific This 1-octet unsigned integer provides more specific informa-
information about the nature of the reported error. Each Error tion about the nature of the reported error. Each Error Code
Code may have one or more Error Subcodes associated with it. may have one or more Error Subcodes associated with it. If no
If no appropriate Error Subcode is defined, then a zero appropriate Error Subcode is defined, then a zero (Unspecific)
(Unspecific) value is used for the Error Subcode field. value is used for the Error Subcode field.
Message Header Error subcodes: Message Header Error subcodes:
1 - Connection Not Synchronized. 1 - Connection Not Synchronized.
2 - Bad Message Length. 2 - Bad Message Length.
3 - Bad Message Type. 3 - Bad Message Type.
OPEN Message Error subcodes: OPEN Message Error subcodes:
1 - Unsupported Version Number. 1 - Unsupported Version Number.
2 - Bad Peer AS. 2 - Bad Peer AS.
3 - Bad BGP Identifier. ' 3 - Bad BGP Identifier.
4 - Unsupported Optional Parameter. 4 - Unsupported Optional Parameter.
5 - Authentication Failure. 5 - Authentication Failure.
6 - Unacceptable Hold Time. 6 - Unacceptable Hold Time.
UPDATE Message Error subcodes: UPDATE Message Error subcodes:
1 - Malformed Attribute List. 1 - Malformed Attribute List.
2 - Unrecognized Well-known Attribute. 2 - Unrecognized Well-known Attribute.
3 - Missing Well-known Attribute. 3 - Missing Well-known Attribute.
4 - Attribute Flags Error. 4 - Attribute Flags Error.
skipping to change at page 19, line 46 skipping to change at page 19, line 46
This variable-length field is used to diagnose the reason for This variable-length field is used to diagnose the reason for
the NOTIFICATION. The contents of the Data field depend upon the NOTIFICATION. The contents of the Data field depend upon
the Error Code and Error Subcode. See Section 6 below for more the Error Code and Error Subcode. See Section 6 below for more
details. details.
Note that the length of the Data field can be determined from Note that the length of the Data field can be determined from
the message Length field by the formula: the message Length field by the formula:
Message Length = 21 + Data Length Message Length = 21 + Data Length
The minimum length of the NOTIFICATION message is 21 octets The minimum length of the NOTIFICATION message is 21 octets (includ-
(including message header). ing message header).
5. Path Attributes 5. Path Attributes
This section discusses the path attributes of the UPDATE message. This section discusses the path attributes of the UPDATE message.
Path attributes fall into four separate categories: Path attributes fall into four separate categories:
1. Well-known mandatory. 1. Well-known mandatory.
2. Well-known discretionary. 2. Well-known discretionary.
3. Optional transitive. 3. Optional transitive.
4. Optional non-transitive. 4. Optional non-transitive.
Well-known attributes must be recognized by all BGP implementations. Well-known attributes must be recognized by all BGP implementations.
Some of these attributes are mandatory and must be included in every Some of these attributes are mandatory and must be included in every
UPDATE message. Others are discretionary and may or may not be sent UPDATE message. Others are discretionary and may or may not be sent
in a particular UPDATE message. in a particular UPDATE message.
All well-known attributes must be passed along (after proper All well-known attributes must be passed along (after proper updat-
updating, if necessary) to other BGP peers. ing, if necessary) to other BGP peers.
In addition to well-known attributes, each path may contain one or In addition to well-known attributes, each path may contain one or
more optional attributes. It is not required or expected that all more optional attributes. It is not required or expected that all
BGP implementations support all optional attributes. The handling of BGP implementations support all optional attributes. The handling of
an unrecognized optional attribute is determined by the setting of an unrecognized optional attribute is determined by the setting of
the Transitive bit in the attribute flags octet. Paths with the Transitive bit in the attribute flags octet. Paths with unrecog-
unrecognized transitive optional attributes should be accepted. If a nized transitive optional attributes should be accepted. If a path
path with unrecognized transitive optional attribute is accepted and with unrecognized transitive optional attribute is accepted and
passed along to other BGP peers, then the unrecognized transitive passed along to other BGP peers, then the unrecognized transitive
optional attribute of that path must be passed along with the path to optional attribute of that path must be passed along with the path to
other BGP peers with the Partial bit in the Attribute Flags octet set other BGP peers with the Partial bit in the Attribute Flags octet set
to 1. If a path with recognized transitive optional attribute is to 1. If a path with recognized transitive optional attribute is
accepted and passed along to other BGP peers and the Partial bit in accepted and passed along to other BGP peers and the Partial bit in
the Attribute Flags octet is set to 1 by some previous AS, it is not the Attribute Flags octet is set to 1 by some previous AS, it is not
set back to 0 by the current AS. Unrecognized non-transitive optional set back to 0 by the current AS. Unrecognized non-transitive optional
attributes must be quietly ignored and not passed along to other BGP attributes must be quietly ignored and not passed along to other BGP
peers. peers.
skipping to change at page 21, line 13 skipping to change at page 21, line 13
by ASs in the path. by ASs in the path.
The sender of an UPDATE message should order path attributes within The sender of an UPDATE message should order path attributes within
the UPDATE message in ascending order of attribute type. The the UPDATE message in ascending order of attribute type. The
receiver of an UPDATE message must be prepared to handle path receiver of an UPDATE message must be prepared to handle path
attributes within the UPDATE message that are out of order. attributes within the UPDATE message that are out of order.
The same attribute cannot appear more than once within the Path The same attribute cannot appear more than once within the Path
Attributes field of a particular UPDATE message. Attributes field of a particular UPDATE message.
The mandatory category refers to a field which must be present in
both IBGP and EBGP exchanges. Attributes classified as optional for
the purpose of the protocol extension mechanism may be purely discre-
tionary, or discretionary, required, or disallowed in certain con-
texts.
attribute EBGP IBGP
ORIGIN mandatory mandatory
AS_PATH mandatory mandatory
NEXT_HOP mandatory mandatory
MULTI_EXIT_DISC discretionary discretionary
LOCAL_PREF disallowed required
ATOMIC_AGGREGATE see section 5.1.6 and 9.1.4
AGGREGATOR discretionary discretionary
5.1 Path Attribute Usage 5.1 Path Attribute Usage
The usage of each BGP path attributes is described in the following The usage of each BGP path attributes is described in the following
clauses. clauses.
5.1.1 ORIGIN 5.1.1 ORIGIN
ORIGIN is a well-known mandatory attribute. The ORIGIN attribute ORIGIN is a well-known mandatory attribute. The ORIGIN attribute
shall be generated by the autonomous system that originates the shall be generated by the autonomous system that originates the asso-
associated routing information. It shall be included in the UPDATE ciated routing information. It shall be included in the UPDATE mes-
messages of all BGP speakers that choose to propagate this sages of all BGP speakers that choose to propagate this information
information to other BGP speakers. to other BGP speakers.
5.1.2 AS_PATH 5.1.2 AS_PATH
AS_PATH is a well-known mandatory attribute. This attribute AS_PATH is a well-known mandatory attribute. This attribute
identifies the autonomous systems through which routing information identifies the autonomous systems through which routing information
carried in this UPDATE message has passed. The components of this carried in this UPDATE message has passed. The components of this
list can be AS_SETs or AS_SEQUENCEs. list can be AS_SETs or AS_SEQUENCEs.
When a BGP speaker propagates a route which it has learned from When a BGP speaker propagates a route which it has learned from
another BGP speaker's UPDATE message, it shall modify the route's another BGP speaker's UPDATE message, it shall modify the route's
AS_PATH attribute based on the location of the BGP speaker to which AS_PATH attribute based on the location of the BGP speaker to which
the route will be sent: the route will be sent:
a) When a given BGP speaker advertises the route to another BGP a) When a given BGP speaker advertises the route to an internal
speaker located in its own autonomous system, the advertising peer, the advertising speaker shall not modify the AS_PATH
speaker shall not modify the AS_PATH attribute associated with the attribute associated with the route.
route.
b) When a given BGP speaker advertises the route to a BGP speaker b) When a given BGP speaker advertises the route to an external
located in a neighboring autonomous system, then the advertising peer, then the advertising speaker shall update the AS_PATH
speaker shall update the AS_PATH attribute as follows: attribute as follows:
1) if the first path segment of the AS_PATH is of type 1) if the first path segment of the AS_PATH is of type
AS_SEQUENCE, the local system shall prepend its own AS number AS_SEQUENCE, the local system shall prepend its own AS number
as the last element of the sequence (put it in the leftmost as the last element of the sequence (put it in the leftmost
position) position)
2) if the first path segment of the AS_PATH is of type AS_SET, 2) if the first path segment of the AS_PATH is of type AS_SET,
the local system shall prepend a new path segment of type the local system shall prepend a new path segment of type
AS_SEQUENCE to the AS_PATH, including its own AS number in that AS_SEQUENCE to the AS_PATH, including its own AS number in that
segment. segment.
When a BGP speaker originates a route then: When a BGP speaker originates a route then:
a) the originating speaker shall include its own AS number in a) the originating speaker shall include its own AS number in
the AS_PATH attribute of all UPDATE messages sent to BGP the AS_PATH attribute of all UPDATE messages sent to an exter-
speakers located in neighboring autonomous systems. (In this nal peer. (In this case, the AS number of the originating
case, the AS number of the originating speaker's autonomous speaker's autonomous system will be the only entry in the
system will be the only entry in the AS_PATH attribute). AS_PATH attribute).
b) the originating speaker shall include an empty AS_PATH b) the originating speaker shall include an empty AS_PATH
attribute in all UPDATE messages sent to BGP speakers located attribute in all UPDATE messages sent to internal peers. (An
in its own autonomous system. (An empty AS_PATH attribute is empty AS_PATH attribute is one whose length field contains the
one whose length field contains the value zero). value zero).
5.1.3 NEXT_HOP 5.1.3 NEXT_HOP
The NEXT_HOP path attribute defines the IP address of the border The NEXT_HOP path attribute defines the IP address of the border
router that should be used as the next hop to the destinations listed router that should be used as the next hop to the destinations listed
in the UPDATE message. If a border router belongs to the same AS as in the UPDATE message. When advertising a NEXT_HOP attribute to an
its peer, then the peer is an internal border router. Otherwise, it external peer, a router may use one of its own interface addresses in
is an external border router. A BGP speaker can advertise any the NEXT_HOP attribute provided the external peer to which the route
internal border router as the next hop provided that the interface is being advertised shares a common subnet with the NEXT_HOP address.
associated with the IP address of this border router (as specified in This is known as a "first party" NEXT_HOP attribute. A BGP speaker
the NEXT_HOP path attribute) shares a common subnet with both the can advertise to an external peer an interface of any internal peer
local and remote BGP speakers. A BGP speaker can advertise any router in the NEXT_HOP attribute provided the external peer to which
external border router as the next hop, provided that the IP address the route is being advertised shares a common subnet with the
of this border router was learned from one of the BGP speaker's NEXT_HOP address. This is known as a "third party" NEXT_HOP
peers, and the interface associated with the IP address of this attribute. A BGP speaker can advertise any external peer router in
border router (as specified in the NEXT_HOP path attribute) shares a the NEXT_HOP attribute provided that the IP address of this border
common subnet with the local and remote BGP speakers. A BGP speaker router was learned from an external peer and the external peer to
needs to be able to support disabling advertisement of external which the route is being advertised shares a common subnet with the
border routers. NEXT_HOP address. This is a second form of "third party" NEXT_HOP
attribute.
Normally the NEXT_HOP attribute is chosen such that the shortest
available path will be taken. A BGP speaker must be able to support
disabling advertisement of third party NEXT_HOP attributes to handle
imperfectly bridged media.
A BGP speaker must never advertise an address of a peer to that peer A BGP speaker must never advertise an address of a peer to that peer
as a NEXT_HOP, for a route that the speaker is originating. A BGP as a NEXT_HOP, for a route that the speaker is originating. A BGP
speaker must never install a route with itself as the next hop. speaker must never install a route with itself as the next hop.
When a BGP speaker advertises the route to a BGP speaker located in When a BGP speaker advertises the route to an internal peer, the
its own autonomous system, the advertising speaker shall not modify advertising speaker should not modify the NEXT_HOP attribute associ-
the NEXT_HOP attribute associated with the route. When a BGP speaker ated with the route. When a BGP speaker receives the route via an
receives the route via an internal link, it may forward packets to internal link, it may forward packets to the NEXT_HOP address if the
the NEXT_HOP address if the address contained in the attribute is on address contained in the attribute is on a common subnet with the
a common subnet with the local and remote BGP speakers. local and remote BGP speakers.
5.1.4 MULTI_EXIT_DISC 5.1.4 MULTI_EXIT_DISC
The MULTI_EXIT_DISC attribute may be used on external (inter-AS) The MULTI_EXIT_DISC attribute may be used on external (inter-AS)
links to discriminate among multiple exit or entry points to the same links to discriminate among multiple exit or entry points to the same
neighboring AS. The value of the MULTI_EXIT_DISC attribute is a four neighboring AS. The value of the MULTI_EXIT_DISC attribute is a four
octet unsigned number which is called a metric. All other factors octet unsigned number which is called a metric. All other factors
being equal, the exit or entry point with lower metric should be being equal, the exit or entry point with lower metric should be pre-
preferred. If received over external links, the MULTI_EXIT_DISC ferred. If received over external links, the MULTI_EXIT_DISC
attribute may be propagated over internal links to other BGP speakers attribute MAY be propagated over internal links to other BGP speakers
within the same AS. The MULTI_EXIT_DISC attribute is never within the same AS. The MULTI_EXIT_DISC attribute received from a
propagated to other BGP speakers in neighboring AS's. neighboring AS MUST NOT be propagated to other neighboring ASs.
A BGP speaker MUST IMPLEMENT a mechanism based on local configuration
which allows the MULTI_EXIT_DISC attribute to be removed from a
route. This MAY be done either prior to or after determining the
degree of preference of the route and performing route selection
(decision process phases 1 and 2).
An implementation MAY also (based on local configuration) alter the
value of the MULTI_EXIT_DISC attribute received over an external
link. If it does so, it shall do so prior to determining the degree
of preference of the route and performing route selection (decision
process phases 1 and 2).
5.1.5 LOCAL_PREF 5.1.5 LOCAL_PREF
LOCAL_PREF is a well-known discretionary attribute that shall be LOCAL_PREF is a well-known mandatory attribute that SHALL be included
included in all UPDATE messages that a given BGP speaker sends to the in all UPDATE messages that a given BGP speaker sends to the other
other BGP speakers located in its own autonomous system. A BGP internal peers. A BGP speaker SHALL calculate the degree of prefer-
speaker shall calculate the degree of preference for each external ence for each external route and include the degree of preference
route and include the degree of preference when advertising a route when advertising a route to its internal peers. The higher degree of
to its internal peers. The higher degree of preference should be preference MUST be preferred. A BGP speaker shall use the degree of
preferred. A BGP speaker shall use the degree of preference learned preference learned via LOCAL_PREF in its decision process (see sec-
via LOCAL_PREF in its decision process (see section 9.1.1). tion 9.1.1).
A BGP speaker shall not include this attribute in UPDATE messages A BGP speaker MUST NOT include this attribute in UPDATE messages that
that it sends to BGP speakers located in a neighboring autonomous it sends to external peers. If it is contained in an UPDATE message
system. If it is contained in an UPDATE message that is received from that is received from an external peer, then this attribute MUST be
a BGP speaker which is not located in the same autonomous system as ignored by the receiving speaker.
the receiving speaker, then this attribute shall be ignored by the
receiving speaker.
5.1.6 ATOMIC_AGGREGATE 5.1.6 ATOMIC_AGGREGATE
ATOMIC_AGGREGATE is a well-known discretionary attribute. If a BGP ATOMIC_AGGREGATE is a well-known discretionary attribute. If a BGP
speaker, when presented with a set of overlapping routes from one of speaker, when presented with a set of overlapping routes from one of
its peers (see 9.1.4), selects the less specific route without its peers (see 9.1.4), selects the less specific route without
selecting the more specific one, then the local system shall attach selecting the more specific one, then the local system MUST attach
the ATOMIC_AGGREGATE attribute to the route when propagating it to the ATOMIC_AGGREGATE attribute to the route when propagating it to
other BGP speakers (if that attribute is not already present in the other BGP speakers (if that attribute is not already present in the
received less specific route). A BGP speaker that receives a route received less specific route). A BGP speaker that receives a route
with the ATOMIC_AGGREGATE attribute shall not remove the attribute with the ATOMIC_AGGREGATE attribute MUST NOT remove the attribute
from the route when propagating it to other speakers. A BGP speaker from the route when propagating it to other speakers. A BGP speaker
that receives a route with the ATOMIC_AGGREGATE attribute shall not that receives a route with the ATOMIC_AGGREGATE attribute MUST NOT
make any NLRI of that route more specific (as defined in 9.1.4) when make any NLRI of that route more specific (as defined in 9.1.4) when
advertising this route to other BGP speakers. A BGP speaker that advertising this route to other BGP speakers. A BGP speaker that
receives a route with the ATOMIC_AGGREGATE attribute needs to be receives a route with the ATOMIC_AGGREGATE attribute needs to be cog-
cognizant of the fact that the actual path to destinations, as nizant of the fact that the actual path to destinations, as specified
specified in the NLRI of the route, while having the loop-free in the NLRI of the route, while having the loop-free property, may
property, may traverse ASs that are not listed in the AS_PATH traverse ASs that are not listed in the AS_PATH attribute.
attribute.
5.1.7 AGGREGATOR 5.1.7 AGGREGATOR
AGGREGATOR is an optional transitive attribute which may be included AGGREGATOR is an optional transitive attribute which may be included
in updates which are formed by aggregation (see Section 9.2.4.2). A in updates which are formed by aggregation (see Section 9.2.4.2). A
BGP speaker which performs route aggregation may add the AGGREGATOR BGP speaker which performs route aggregation may add the AGGREGATOR
attribute which shall contain its own AS number and IP address. attribute which shall contain its own AS number and IP address.
6. BGP Error Handling. 6. BGP Error Handling.
This section describes actions to be taken when errors are detected This section describes actions to be taken when errors are detected
while processing BGP messages. while processing BGP messages.
When any of the conditions described here are detected, a When any of the conditions described here are detected, a NOTIFICA-
NOTIFICATION message with the indicated Error Code, Error Subcode, TION message with the indicated Error Code, Error Subcode, and Data
and Data fields is sent, and the BGP connection is closed. If no fields is sent, and the BGP connection is closed. If no Error Sub-
Error Subcode is specified, then a zero must be used. code is specified, then a zero must be used.
The phrase "the BGP connection is closed" means that the transport The phrase "the BGP connection is closed" means that the transport
protocol connection has been closed and that all resources for that protocol connection has been closed and that all resources for that
BGP connection have been deallocated. Routing table entries BGP connection have been deallocated. Routing table entries associ-
associated with the remote peer are marked as invalid. The fact that ated with the remote peer are marked as invalid. The fact that the
the routes have become invalid is passed to other BGP peers before routes have become invalid is passed to other BGP peers before the
the routes are deleted from the system. routes are deleted from the system.
Unless specified explicitly, the Data field of the NOTIFICATION Unless specified explicitly, the Data field of the NOTIFICATION mes-
message that is sent to indicate an error is empty. sage that is sent to indicate an error is empty.
6.1 Message Header error handling. 6.1 Message Header error handling.
All errors detected while processing the Message Header are indicated All errors detected while processing the Message Header are indicated
by sending the NOTIFICATION message with Error Code Message Header by sending the NOTIFICATION message with Error Code Message Header
Error. The Error Subcode elaborates on the specific nature of the Error. The Error Subcode elaborates on the specific nature of the
error. error.
The expected value of the Marker field of the message header is all The expected value of the Marker field of the message header is all
ones if the message type is OPEN. The expected value of the Marker ones if the message type is OPEN. The expected value of the Marker
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Authentication Information in the BGP OPEN message is present). If Authentication Information in the BGP OPEN message is present). If
the Marker field of the message header is not the expected one, then the Marker field of the message header is not the expected one, then
a synchronization error has occurred and the Error Subcode is set to a synchronization error has occurred and the Error Subcode is set to
Connection Not Synchronized. Connection Not Synchronized.
If the Length field of the message header is less than 19 or greater If the Length field of the message header is less than 19 or greater
than 4096, or if the Length field of an OPEN message is less than than 4096, or if the Length field of an OPEN message is less than
the minimum length of the OPEN message, or if the Length field of an the minimum length of the OPEN message, or if the Length field of an
UPDATE message is less than the minimum length of the UPDATE message, UPDATE message is less than the minimum length of the UPDATE message,
or if the Length field of a KEEPALIVE message is not equal to 19, or or if the Length field of a KEEPALIVE message is not equal to 19, or
if the Length field of a NOTIFICATION message is less than the if the Length field of a NOTIFICATION message is less than the mini-
minimum length of the NOTIFICATION message, then the Error Subcode is mum length of the NOTIFICATION message, then the Error Subcode is set
set to Bad Message Length. The Data field contains the erroneous to Bad Message Length. The Data field contains the erroneous Length
Length field. field.
If the Type field of the message header is not recognized, then the If the Type field of the message header is not recognized, then the
Error Subcode is set to Bad Message Type. The Data field contains Error Subcode is set to Bad Message Type. The Data field contains
the erroneous Type field. the erroneous Type field.
6.2 OPEN message error handling. 6.2 OPEN message error handling.
All errors detected while processing the OPEN message are indicated All errors detected while processing the OPEN message are indicated
by sending the NOTIFICATION message with Error Code OPEN Message by sending the NOTIFICATION message with Error Code OPEN Message
Error. The Error Subcode elaborates on the specific nature of the Error. The Error Subcode elaborates on the specific nature of the
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integer, which indicates the largest locally supported version number integer, which indicates the largest locally supported version number
less than the version the remote BGP peer bid (as indicated in the less than the version the remote BGP peer bid (as indicated in the
received OPEN message). received OPEN message).
If the Autonomous System field of the OPEN message is unacceptable, If the Autonomous System field of the OPEN message is unacceptable,
then the Error Subcode is set to Bad Peer AS. The determination of then the Error Subcode is set to Bad Peer AS. The determination of
acceptable Autonomous System numbers is outside the scope of this acceptable Autonomous System numbers is outside the scope of this
protocol. protocol.
If the Hold Time field of the OPEN message is unacceptable, then the If the Hold Time field of the OPEN message is unacceptable, then the
Error Subcode MUST be set to Unacceptable Hold Time. An Error Subcode MUST be set to Unacceptable Hold Time. An implementa-
implementation MUST reject Hold Time values of one or two seconds. tion MUST reject Hold Time values of one or two seconds. An imple-
An implementation MAY reject any proposed Hold Time. An mentation MAY reject any proposed Hold Time. An implementation which
implementation which accepts a Hold Time MUST use the negotiated accepts a Hold Time MUST use the negotiated value for the Hold Time.
value for the Hold Time.
If the BGP Identifier field of the OPEN message is syntactically If the BGP Identifier field of the OPEN message is syntactically
incorrect, then the Error Subcode is set to Bad BGP Identifier. incorrect, then the Error Subcode is set to Bad BGP Identifier. Syn-
Syntactic correctness means that the BGP Identifier field represents tactic correctness means that the BGP Identifier field represents a
a valid IP host address. valid IP host address.
If one of the Optional Parameters in the OPEN message is not If one of the Optional Parameters in the OPEN message is not recog-
recognized, then the Error Subcode is set to Unsupported Optional nized, then the Error Subcode is set to Unsupported Optional
Parameters. Parameters.
If the OPEN message carries Authentication Information (as an If the OPEN message carries Authentication Information (as an
Optional Parameter), then the corresponding authentication procedure Optional Parameter), then the corresponding authentication procedure
is invoked. If the authentication procedure (based on Authentication is invoked. If the authentication procedure (based on Authentication
Code and Authentication Data) fails, then the Error Subcode is set to Code and Authentication Data) fails, then the Error Subcode is set to
Authentication Failure. Authentication Failure.
If the OPEN message carries any other Optional Parameter (other than If the OPEN message carries any other Optional Parameter (other than
Authentication Information), and the local system doesn't recognize Authentication Information), and the local system doesn't recognize
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Attribute Length + 23 exceeds the message Length), then the Error Attribute Length + 23 exceeds the message Length), then the Error
Subcode is set to Malformed Attribute List. Subcode is set to Malformed Attribute List.
If any recognized attribute has Attribute Flags that conflict with If any recognized attribute has Attribute Flags that conflict with
the Attribute Type Code, then the Error Subcode is set to Attribute the Attribute Type Code, then the Error Subcode is set to Attribute
Flags Error. The Data field contains the erroneous attribute (type, Flags Error. The Data field contains the erroneous attribute (type,
length and value). length and value).
If any recognized attribute has Attribute Length that conflicts with If any recognized attribute has Attribute Length that conflicts with
the expected length (based on the attribute type code), then the the expected length (based on the attribute type code), then the
Error Subcode is set to Attribute Length Error. The Data field Error Subcode is set to Attribute Length Error. The Data field con-
contains the erroneous attribute (type, length and value). tains the erroneous attribute (type, length and value).
If any of the mandatory well-known attributes are not present, then If any of the mandatory well-known attributes are not present, then
the Error Subcode is set to Missing Well-known Attribute. The Data the Error Subcode is set to Missing Well-known Attribute. The Data
field contains the Attribute Type Code of the missing well-known field contains the Attribute Type Code of the missing well-known
attribute. attribute.
If any of the mandatory well-known attributes are not recognized, If any of the mandatory well-known attributes are not recognized,
then the Error Subcode is set to Unrecognized Well-known Attribute. then the Error Subcode is set to Unrecognized Well-known Attribute.
The Data field contains the unrecognized attribute (type, length and The Data field contains the unrecognized attribute (type, length and
value). value).
If the ORIGIN attribute has an undefined value, then the Error If the ORIGIN attribute has an undefined value, then the Error Sub-
Subcode is set to Invalid Origin Attribute. The Data field contains code is set to Invalid Origin Attribute. The Data field contains the
the unrecognized attribute (type, length and value). unrecognized attribute (type, length and value).
If the NEXT_HOP attribute field is syntactically incorrect, then the If the NEXT_HOP attribute field is syntactically incorrect, then the
Error Subcode is set to Invalid NEXT_HOP Attribute. The Data field Error Subcode is set to Invalid NEXT_HOP Attribute. The Data field
contains the incorrect attribute (type, length and value). Syntactic contains the incorrect attribute (type, length and value). Syntactic
correctness means that the NEXT_HOP attribute represents a valid IP correctness means that the NEXT_HOP attribute represents a valid IP
host address. Semantic correctness applies only to the external BGP host address. Semantic correctness applies only to the external BGP
links. It means that the interface associated with the IP address, as links. It means that the interface associated with the IP address, as
specified in the NEXT_HOP attribute, shares a common subnet with the specified in the NEXT_HOP attribute, shares a common subnet with the
receiving BGP speaker and is not the IP address of the receiving BGP receiving BGP speaker and is not the IP address of the receiving BGP
speaker. If the NEXT_HOP attribute is semantically incorrect, the speaker. If the NEXT_HOP attribute is semantically incorrect, the
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specified in the AS_PATH attribute), and checking that the autonomous specified in the AS_PATH attribute), and checking that the autonomous
system number of the local system does not appear in the AS path. If system number of the local system does not appear in the AS path. If
the autonomous system number appears in the AS path the route may be the autonomous system number appears in the AS path the route may be
stored in the Adj-RIB-In, but unless the router is configured to stored in the Adj-RIB-In, but unless the router is configured to
accept routes with its own autonomous system in the AS path, the accept routes with its own autonomous system in the AS path, the
route shall not be passed to the BGP Decision Process. Operations of route shall not be passed to the BGP Decision Process. Operations of
a router that is configured to accept routes with its own autonomous a router that is configured to accept routes with its own autonomous
system number in the AS path are outside the scope of this document. system number in the AS path are outside the scope of this document.
If an optional attribute is recognized, then the value of this If an optional attribute is recognized, then the value of this
attribute is checked. If an error is detected, the attribute is attribute is checked. If an error is detected, the attribute is dis-
discarded, and the Error Subcode is set to Optional Attribute Error. carded, and the Error Subcode is set to Optional Attribute Error.
The Data field contains the attribute (type, length and value). The Data field contains the attribute (type, length and value).
If any attribute appears more than once in the UPDATE message, then If any attribute appears more than once in the UPDATE message, then
the Error Subcode is set to Malformed Attribute List. the Error Subcode is set to Malformed Attribute List.
The NLRI field in the UPDATE message is checked for syntactic The NLRI field in the UPDATE message is checked for syntactic valid-
validity. If the field is syntactically incorrect, then the Error ity. If the field is syntactically incorrect, then the Error Subcode
Subcode is set to Invalid Network Field. is set to Invalid Network Field.
6.4 NOTIFICATION message error handling. 6.4 NOTIFICATION message error handling.
If a peer sends a NOTIFICATION message, and there is an error in that If a peer sends a NOTIFICATION message, and there is an error in that
message, there is unfortunately no means of reporting this error via message, there is unfortunately no means of reporting this error via
a subsequent NOTIFICATION message. Any such error, such as an a subsequent NOTIFICATION message. Any such error, such as an unrec-
unrecognized Error Code or Error Subcode, should be noticed, logged ognized Error Code or Error Subcode, should be noticed, logged
locally, and brought to the attention of the administration of the locally, and brought to the attention of the administration of the
peer. The means to do this, however, lies outside the scope of this peer. The means to do this, however, lies outside the scope of this
document. document.
6.5 Hold Timer Expired error handling. 6.5 Hold Timer Expired error handling.
If a system does not receive successive KEEPALIVE and/or UPDATE If a system does not receive successive KEEPALIVE and/or UPDATE
and/or NOTIFICATION messages within the period specified in the Hold and/or NOTIFICATION messages within the period specified in the Hold
Time field of the OPEN message, then the NOTIFICATION message with Time field of the OPEN message, then the NOTIFICATION message with
Hold Timer Expired Error Code must be sent and the BGP connection Hold Timer Expired Error Code must be sent and the BGP connection
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6.7 Cease. 6.7 Cease.
In absence of any fatal errors (that are indicated in this section), In absence of any fatal errors (that are indicated in this section),
a BGP peer may choose at any given time to close its BGP connection a BGP peer may choose at any given time to close its BGP connection
by sending the NOTIFICATION message with Error Code Cease. However, by sending the NOTIFICATION message with Error Code Cease. However,
the Cease NOTIFICATION message must not be used when a fatal error the Cease NOTIFICATION message must not be used when a fatal error
indicated by this section does exist. indicated by this section does exist.
6.8 Connection collision detection. 6.8 Connection collision detection.
If a pair of BGP speakers try simultaneously to establish a TCP If a pair of BGP speakers try simultaneously to establish a TCP con-
connection to each other, then two parallel connections between this nection to each other, then two parallel connections between this
pair of speakers might well be formed. We refer to this situation as pair of speakers might well be formed. We refer to this situation as
connection collision. Clearly, one of these connections must be connection collision. Clearly, one of these connections must be
closed. closed.
Based on the value of the BGP Identifier a convention is established Based on the value of the BGP Identifier a convention is established
for detecting which BGP connection is to be preserved when a for detecting which BGP connection is to be preserved when a colli-
collision does occur. The convention is to compare the BGP sion does occur. The convention is to compare the BGP Identifiers of
Identifiers of the peers involved in the collision and to retain only the peers involved in the collision and to retain only the connection
the connection initiated by the BGP speaker with the higher-valued initiated by the BGP speaker with the higher-valued BGP Identifier.
BGP Identifier.
Upon receipt of an OPEN message, the local system must examine all of Upon receipt of an OPEN message, the local system must examine all of
its connections that are in the OpenConfirm state. A BGP speaker may its connections that are in the OpenConfirm state. A BGP speaker may
also examine connections in an OpenSent state if it knows the BGP also examine connections in an OpenSent state if it knows the BGP
Identifier of the peer by means outside of the protocol. If among Identifier of the peer by means outside of the protocol. If among
these connections there is a connection to a remote BGP speaker whose these connections there is a connection to a remote BGP speaker whose
BGP Identifier equals the one in the OPEN message, then the local BGP Identifier equals the one in the OPEN message, then the local
system performs the following collision resolution procedure: system performs the following collision resolution procedure:
1. The BGP Identifier of the local system is compared to the BGP 1. The BGP Identifier of the local system is compared to the BGP
Identifier of the remote system (as specified in the OPEN Identifier of the remote system (as specified in the OPEN mes-
message). sage).
2. If the value of the local BGP Identifier is less than the 2. If the value of the local BGP Identifier is less than the
remote one, the local system closes BGP connection that already remote one, the local system closes BGP connection that already
exists (the one that is already in the OpenConfirm state), and exists (the one that is already in the OpenConfirm state), and
accepts BGP connection initiated by the remote system. accepts BGP connection initiated by the remote system.
3. Otherwise, the local system closes newly created BGP connection 3. Otherwise, the local system closes newly created BGP connection
(the one associated with the newly received OPEN message), and (the one associated with the newly received OPEN message), and
continues to use the existing one (the one that is already in the continues to use the existing one (the one that is already in the
OpenConfirm state). OpenConfirm state).
Comparing BGP Identifiers is done by treating them as (4-octet Comparing BGP Identifiers is done by treating them as (4-octet
long) unsigned integers. long) unsigned integers.
A connection collision with an existing BGP connection that is in A connection collision with an existing BGP connection that is in
Established states causes unconditional closing of the newly Established states causes unconditional closing of the newly cre-
created connection. Note that a connection collision cannot be ated connection. Note that a connection collision cannot be
detected with connections that are in Idle, or Connect, or Active detected with connections that are in Idle, or Connect, or Active
states. states.
Closing the BGP connection (that results from the collision Closing the BGP connection (that results from the collision reso-
resolution procedure) is accomplished by sending the NOTIFICATION lution procedure) is accomplished by sending the NOTIFICATION mes-
message with the Error Code Cease. sage with the Error Code Cease.
7. BGP Version Negotiation. 7. BGP Version Negotiation.
BGP speakers may negotiate the version of the protocol by making BGP speakers may negotiate the version of the protocol by making mul-
multiple attempts to open a BGP connection, starting with the highest tiple attempts to open a BGP connection, starting with the highest
version number each supports. If an open attempt fails with an Error version number each supports. If an open attempt fails with an Error
Code OPEN Message Error, and an Error Subcode Unsupported Version Code OPEN Message Error, and an Error Subcode Unsupported Version
Number, then the BGP speaker has available the version number it Number, then the BGP speaker has available the version number it
tried, the version number its peer tried, the version number passed tried, the version number its peer tried, the version number passed
by its peer in the NOTIFICATION message, and the version numbers that by its peer in the NOTIFICATION message, and the version numbers that
it supports. If the two peers do support one or more common it supports. If the two peers do support one or more common ver-
versions, then this will allow them to rapidly determine the highest sions, then this will allow them to rapidly determine the highest
common version. In order to support BGP version negotiation, future common version. In order to support BGP version negotiation, future
versions of BGP must retain the format of the OPEN and NOTIFICATION versions of BGP must retain the format of the OPEN and NOTIFICATION
messages. messages.
8. BGP Finite State machine. 8. BGP Finite State machine.
This section specifies BGP operation in terms of a Finite State This section specifies BGP operation in terms of a Finite State
Machine (FSM). Following is a brief summary and overview of BGP Machine (FSM). Following is a brief summary and overview of BGP
operations by state as determined by this FSM. A condensed version operations by state as determined by this FSM. A condensed version
of the BGP FSM is found in Appendix 1. of the BGP FSM is found in Appendix 1.
Initially BGP is in the Idle state. Initially BGP is in the Idle state.
Idle state: Idle state:
In this state BGP refuses all incoming BGP connections. No In this state BGP refuses all incoming BGP connections. No
resources are allocated to the peer. In response to the Start resources are allocated to the peer. In response to the Start
event (initiated by either system or operator) the local system event (initiated by either system or operator) the local system
initializes all BGP resources, starts the ConnectRetry timer, initializes all BGP resources, starts the ConnectRetry timer,
initiates a transport connection to other BGP peer, while initiates a transport connection to other BGP peer, while lis-
listening for connection that may be initiated by the remote tening for connection that may be initiated by the remote BGP
BGP peer, and changes its state to Connect. The exact value of peer, and changes its state to Connect. The exact value of the
the ConnectRetry timer is a local matter, but should be ConnectRetry timer is a local matter, but should be suffi-
sufficiently large to allow TCP initialization. ciently large to allow TCP initialization.
If a BGP speaker detects an error, it shuts down the connection If a BGP speaker detects an error, it shuts down the connection
and changes its state to Idle. Getting out of the Idle state and changes its state to Idle. Getting out of the Idle state
requires generation of the Start event. If such an event is requires generation of the Start event. If such an event is
generated automatically, then persistent BGP errors may result generated automatically, then persistent BGP errors may result
in persistent flapping of the speaker. To avoid such a in persistent flapping of the speaker. To avoid such a condi-
condition it is recommended that Start events should not be tion it is recommended that Start events should not be gener-
generated immediately for a peer that was previously ated immediately for a peer that was previously transitioned to
transitioned to Idle due to an error. For a peer that was Idle due to an error. For a peer that was previously transi-
previously transitioned to Idle due to an error, the time tioned to Idle due to an error, the time between consecutive
between consecutive generation of Start events, if such events generation of Start events, if such events are generated auto-
are generated automatically, shall exponentially increase. The matically, shall exponentially increase. The value of the ini-
value of the initial timer shall be 60 seconds. The time shall tial timer shall be 60 seconds. The time shall be doubled for
be doubled for each consecutive retry. each consecutive retry.
Any other event received in the Idle state is ignored. Any other event received in the Idle state is ignored.
Connect state: Connect state:
In this state BGP is waiting for the transport protocol In this state BGP is waiting for the transport protocol connec-
connection to be completed. tion to be completed.
If the transport protocol connection succeeds, the local system If the transport protocol connection succeeds, the local system
clears the ConnectRetry timer, completes initialization, sends clears the ConnectRetry timer, completes initialization, sends
an OPEN message to its peer, and changes its state to OpenSent. an OPEN message to its peer, and changes its state to OpenSent.
If the transport protocol connect fails (e.g., retransmission If the transport protocol connect fails (e.g., retransmission
timeout), the local system restarts the ConnectRetry timer, timeout), the local system restarts the ConnectRetry timer,
continues to listen for a connection that may be initiated by continues to listen for a connection that may be initiated by
the remote BGP peer, and changes its state to Active state. the remote BGP peer, and changes its state to Active state.
In response to the ConnectRetry timer expired event, the local In response to the ConnectRetry timer expired event, the local
system restarts the ConnectRetry timer, initiates a transport system restarts the ConnectRetry timer, initiates a transport
connection to other BGP peer, continues to listen for a connection to other BGP peer, continues to listen for a connec-
connection that may be initiated by the remote BGP peer, and tion that may be initiated by the remote BGP peer, and stays in
stays in the Connect state. the Connect state.
Start event is ignored in the Active state. Start event is ignored in the Active state.
In response to any other event (initiated by either system or In response to any other event (initiated by either system or
operator), the local system releases all BGP resources operator), the local system releases all BGP resources associ-
associated with this connection and changes its state to Idle. ated with this connection and changes its state to Idle.
Active state: Active state:
In this state BGP is trying to acquire a peer by initiating a In this state BGP is trying to acquire a peer by initiating a
transport protocol connection. transport protocol connection.
If the transport protocol connection succeeds, the local system If the transport protocol connection succeeds, the local system
clears the ConnectRetry timer, completes initialization, sends clears the ConnectRetry timer, completes initialization, sends
an OPEN message to its peer, sets its Hold Timer to a large an OPEN message to its peer, sets its Hold Timer to a large
value, and changes its state to OpenSent. A Hold Timer value value, and changes its state to OpenSent. A Hold Timer value
of 4 minutes is suggested. of 4 minutes is suggested.
In response to the ConnectRetry timer expired event, the local In response to the ConnectRetry timer expired event, the local
system restarts the ConnectRetry timer, initiates a transport system restarts the ConnectRetry timer, initiates a transport
connection to other BGP peer, continues to listen for a connection to other BGP peer, continues to listen for a connec-
connection that may be initiated by the remote BGP peer, and tion that may be initiated by the remote BGP peer, and changes
changes its state to Connect. its state to Connect.
If the local system detects that a remote peer is trying to If the local system detects that a remote peer is trying to
establish BGP connection to it, and the IP address of the establish BGP connection to it, and the IP address of the
remote peer is not an expected one, the local system restarts remote peer is not an expected one, the local system restarts
the ConnectRetry timer, rejects the attempted connection, the ConnectRetry timer, rejects the attempted connection, con-
continues to listen for a connection that may be initiated by tinues to listen for a connection that may be initiated by the
the remote BGP peer, and stays in the Active state. remote BGP peer, and stays in the Active state.
Start event is ignored in the Active state. Start event is ignored in the Active state.
In response to any other event (initiated by either system or In response to any other event (initiated by either system or
operator), the local system releases all BGP resources operator), the local system releases all BGP resources associ-
associated with this connection and changes its state to Idle. ated with this connection and changes its state to Idle.
OpenSent state: OpenSent state:
In this state BGP waits for an OPEN message from its peer. In this state BGP waits for an OPEN message from its peer.
When an OPEN message is received, all fields are checked for When an OPEN message is received, all fields are checked for
correctness. If the BGP message header checking or OPEN correctness. If the BGP message header checking or OPEN mes-
message checking detects an error (see Section 6.2), or a sage checking detects an error (see Section 6.2), or a connec-
connection collision (see Section 6.8) the local system sends a tion collision (see Section 6.8) the local system sends a NOTI-
NOTIFICATION message and changes its state to Idle. FICATION message and changes its state to Idle.
If there are no errors in the OPEN message, BGP sends a If there are no errors in the OPEN message, BGP sends a
KEEPALIVE message and sets a KeepAlive timer. The Hold Timer, KEEPALIVE message and sets a KeepAlive timer. The Hold Timer,
which was originally set to a large value (see above), is which was originally set to a large value (see above), is
replaced with the negotiated Hold Time value (see section 4.2). replaced with the negotiated Hold Time value (see section 4.2).
If the negotiated Hold Time value is zero, then the Hold Time If the negotiated Hold Time value is zero, then the Hold Time
timer and KeepAlive timers are not started. If the value of timer and KeepAlive timers are not started. If the value of
the Autonomous System field is the same as the local Autonomous the Autonomous System field is the same as the local Autonomous
System number, then the connection is an "internal" connection; System number, then the connection is an "internal" connection;
otherwise, it is "external". (This will effect UPDATE otherwise, it is "external". (This will effect UPDATE process-
processing as described below.) Finally, the state is changed ing as described below.) Finally, the state is changed to
to OpenConfirm. OpenConfirm.
If a disconnect notification is received from the underlying If a disconnect notification is received from the underlying
transport protocol, the local system closes the BGP connection, transport protocol, the local system closes the BGP connection,
restarts the ConnectRetry timer, while continue listening for restarts the ConnectRetry timer, while continue listening for
connection that may be initiated by the remote BGP peer, and connection that may be initiated by the remote BGP peer, and
goes into the Active state. goes into the Active state.
If the Hold Timer expires, the local system sends NOTIFICATION If the Hold Timer expires, the local system sends NOTIFICATION
message with error code Hold Timer Expired and changes its message with error code Hold Timer Expired and changes its
state to Idle. state to Idle.
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In response to any other event the local system sends In response to any other event the local system sends
NOTIFICATION message with Error Code Finite State Machine Error NOTIFICATION message with Error Code Finite State Machine Error
and changes its state to Idle. and changes its state to Idle.
Whenever BGP changes its state from OpenSent to Idle, it closes Whenever BGP changes its state from OpenSent to Idle, it closes
the BGP (and transport-level) connection and releases all the BGP (and transport-level) connection and releases all
resources associated with that connection. resources associated with that connection.
OpenConfirm state: OpenConfirm state:
In this state BGP waits for a KEEPALIVE or NOTIFICATION In this state BGP waits for a KEEPALIVE or NOTIFICATION mes-
message. sage.
If the local system receives a KEEPALIVE message, it changes If the local system receives a KEEPALIVE message, it changes
its state to Established. its state to Established.
If the Hold Timer expires before a KEEPALIVE message is If the Hold Timer expires before a KEEPALIVE message is
received, the local system sends NOTIFICATION message with received, the local system sends NOTIFICATION message with
error code Hold Timer Expired and changes its state to Idle. error code Hold Timer Expired and changes its state to Idle.
If the local system receives a NOTIFICATION message, it changes If the local system receives a NOTIFICATION message, it changes
its state to Idle. its state to Idle.
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If a disconnect notification is received from the underlying If a disconnect notification is received from the underlying
transport protocol, the local system changes its state to Idle. transport protocol, the local system changes its state to Idle.
In response to the Stop event (initiated by either system or In response to the Stop event (initiated by either system or
operator) the local system sends NOTIFICATION message with operator) the local system sends NOTIFICATION message with
Error Code Cease and changes its state to Idle. Error Code Cease and changes its state to Idle.
Start event is ignored in the OpenConfirm state. Start event is ignored in the OpenConfirm state.
In response to any other event the local system sends In response to any other event the local system sends NOTIFICA-
NOTIFICATION message with Error Code Finite State Machine Error TION message with Error Code Finite State Machine Error and
and changes its state to Idle. changes its state to Idle.
Whenever BGP changes its state from OpenConfirm to Idle, it Whenever BGP changes its state from OpenConfirm to Idle, it
closes the BGP (and transport-level) connection and releases closes the BGP (and transport-level) connection and releases
all resources associated with that connection. all resources associated with that connection.
Established state: Established state:
In the Established state BGP can exchange UPDATE, NOTIFICATION, In the Established state BGP can exchange UPDATE, NOTIFICATION,
and KEEPALIVE messages with its peer. and KEEPALIVE messages with its peer.
skipping to change at page 34, line 35 skipping to change at page 35, line 18
its state to Idle. its state to Idle.
If the local system receives an UPDATE message and the UPDATE If the local system receives an UPDATE message and the UPDATE
message error handling procedure (see Section 6.3) detects an message error handling procedure (see Section 6.3) detects an
error, the local system sends a NOTIFICATION message and error, the local system sends a NOTIFICATION message and
changes its state to Idle. changes its state to Idle.
If a disconnect notification is received from the underlying If a disconnect notification is received from the underlying
transport protocol, the local system changes its state to Idle. transport protocol, the local system changes its state to Idle.
If the Hold Timer expires, the local system sends a If the Hold Timer expires, the local system sends a NOTIFICA-
NOTIFICATION message with Error Code Hold Timer Expired and TION message with Error Code Hold Timer Expired and changes its
changes its state to Idle. state to Idle.
If the KeepAlive timer expires, the local system sends a If the KeepAlive timer expires, the local system sends a
KEEPALIVE message and restarts its KeepAlive timer. KEEPALIVE message and restarts its KeepAlive timer.
Each time the local system sends a KEEPALIVE or UPDATE message, Each time the local system sends a KEEPALIVE or UPDATE message,
it restarts its KeepAlive timer, unless the negotiated Hold it restarts its KeepAlive timer, unless the negotiated Hold
Time value is zero. Time value is zero.
In response to the Stop event (initiated by either system or In response to the Stop event (initiated by either system or
operator), the local system sends a NOTIFICATION message with operator), the local system sends a NOTIFICATION message with
Error Code Cease and changes its state to Idle. Error Code Cease and changes its state to Idle.
Start event is ignored in the Established state. Start event is ignored in the Established state.
In response to any other event, the local system sends In response to any other event, the local system sends NOTIFI-
NOTIFICATION message with Error Code Finite State Machine Error CATION message with Error Code Finite State Machine Error and
and changes its state to Idle. changes its state to Idle.
Whenever BGP changes its state from Established to Idle, it Whenever BGP changes its state from Established to Idle, it
closes the BGP (and transport-level) connection, releases all closes the BGP (and transport-level) connection, releases all
resources associated with that connection, and deletes all resources associated with that connection, and deletes all
routes derived from that connection. routes derived from that connection.
9. UPDATE Message Handling 9. UPDATE Message Handling
An UPDATE message may be received only in the Established state. An UPDATE message may be received only in the Established state.
When an UPDATE message is received, each field is checked for When an UPDATE message is received, each field is checked for valid-
validity as specified in Section 6.3. ity as specified in Section 6.3.
If an optional non-transitive attribute is unrecognized, it is If an optional non-transitive attribute is unrecognized, it is qui-
quietly ignored. If an optional transitive attribute is etly ignored. If an optional transitive attribute is unrecognized,
unrecognized, the Partial bit (the third high-order bit) in the the Partial bit (the third high-order bit) in the attribute flags
attribute flags octet is set to 1, and the attribute is retained for octet is set to 1, and the attribute is retained for propagation to
propagation to other BGP speakers. other BGP speakers.
If an optional attribute is recognized, and has a valid value, then, If an optional attribute is recognized, and has a valid value, then,
depending on the type of the optional attribute, it is processed depending on the type of the optional attribute, it is processed
locally, retained, and updated, if necessary, for possible locally, retained, and updated, if necessary, for possible propaga-
propagation to other BGP speakers. tion to other BGP speakers.
If the UPDATE message contains a non-empty WITHDRAWN ROUTES field, If the UPDATE message contains a non-empty WITHDRAWN ROUTES field,
the previously advertised routes whose destinations (expressed as IP the previously advertised routes whose destinations (expressed as IP
prefixes) contained in this field shall be removed from the Adj-RIB- prefixes) contained in this field shall be removed from the Adj-RIB-
In. This BGP speaker shall run its Decision Process since the In. This BGP speaker shall run its Decision Process since the previ-
previously advertised route is not longer available for use. ously advertised route is not longer available for use.
If the UPDATE message contains a feasible route, it shall be placed If the UPDATE message contains a feasible route, it shall be placed
in the appropriate Adj-RIB-In, and the following additional actions in the appropriate Adj-RIB-In, and the following additional actions
shall be taken: shall be taken:
i) If its Network Layer Reachability Information (NLRI) is identical i) If its Network Layer Reachability Information (NLRI) is identical
to the one of a route currently stored in the Adj-RIB-In, then the to the one of a route currently stored in the Adj-RIB-In, then the
new route shall replace the older route in the Adj-RIB-In, thus new route shall replace the older route in the Adj-RIB-In, thus
implicitly withdrawing the older route from service. The BGP speaker implicitly withdrawing the older route from service. The BGP speaker
shall run its Decision Process since the older route is no longer shall run its Decision Process since the older route is no longer
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v) If the new route is an overlapping route that is less specific v) If the new route is an overlapping route that is less specific
(see 9.1.4) than an earlier route contained in the Adj-RIB-In, the (see 9.1.4) than an earlier route contained in the Adj-RIB-In, the
BGP speaker shall run its Decision Process on the set of destinations BGP speaker shall run its Decision Process on the set of destinations
described only by the less specific route. described only by the less specific route.
9.1 Decision Process 9.1 Decision Process
The Decision Process selects routes for subsequent advertisement by The Decision Process selects routes for subsequent advertisement by
applying the policies in the local Policy Information Base (PIB) to applying the policies in the local Policy Information Base (PIB) to
the routes stored in its Adj-RIB-In. The output of the Decision the routes stored in its Adj-RIB-In. The output of the Decision Pro-
Process is the set of routes that will be advertised to all peers; cess is the set of routes that will be advertised to all peers; the
the selected routes will be stored in the local speaker's Adj-RIB- selected routes will be stored in the local speaker's Adj-RIB-Out.
Out.
The selection process is formalized by defining a function that takes The selection process is formalized by defining a function that takes
the attribute of a given route as an argument and returns a non- the attribute of a given route as an argument and returns a non-
negative integer denoting the degree of preference for the route. negative integer denoting the degree of preference for the route.
The function that calculates the degree of preference for a given The function that calculates the degree of preference for a given
route shall not use as its inputs any of the following: the existence route shall not use as its inputs any of the following: the existence
of other routes, the non-existence of other routes, or the path of other routes, the non-existence of other routes, or the path
attributes of other routes. Route selection then consists of attributes of other routes. Route selection then consists of individ-
individual application of the degree of preference function to each ual application of the degree of preference function to each feasible
feasible route, followed by the choice of the one with the highest route, followed by the choice of the one with the highest degree of
degree of preference. preference.
The Decision Process operates on routes contained in each Adj-RIB-In, The Decision Process operates on routes contained in each Adj-RIB-In,
and is responsible for: and is responsible for:
- selection of routes to be advertised to BGP speakers located in - selection of routes to be advertised to internal peers
the local speaker's autonomous system
- selection of routes to be advertised to external peers
- selection of routes to be advertised to BGP speakers located in
neighboring autonomous systems
- route aggregation and route information reduction - route aggregation and route information reduction
The Decision Process takes place in three distinct phases, each The Decision Process takes place in three distinct phases, each trig-
triggered by a different event: gered by a different event:
a) Phase 1 is responsible for calculating the degree of preference a) Phase 1 is responsible for calculating the degree of preference
for each route received from a BGP speaker located in a for each route received from an external peer, and for advertising
neighboring autonomous system, and for advertising to the other to the other internal peers the routes that have the highest
BGP speakers in the local autonomous system the routes that have degree of preference for each distinct destination.
the highest degree of preference for each distinct destination.
b) Phase 2 is invoked on completion of phase 1. It is responsible b) Phase 2 is invoked on completion of phase 1. It is responsible
for choosing the best route out of all those available for each for choosing the best route out of all those available for each
distinct destination, and for installing each chosen route into distinct destination, and for installing each chosen route into
the appropriate Loc-RIB. the appropriate Loc-RIB.
c) Phase 3 is invoked after the Loc-RIB has been modified. It is c) Phase 3 is invoked after the Loc-RIB has been modified. It is
responsible for disseminating routes in the Loc-RIB to each peer responsible for disseminating routes in the Loc-RIB to each exter-
located in a neighboring autonomous system, according to the nal peer, according to the policies contained in the PIB. Route
policies contained in the PIB. Route aggregation and information aggregation and information reduction can optionally be performed
reduction can optionally be performed within this phase. within this phase.
9.1.1 Phase 1: Calculation of Degree of Preference 9.1.1 Phase 1: Calculation of Degree of Preference
The Phase 1 decision function shall be invoked whenever the local BGP The Phase 1 decision function shall be invoked whenever the local BGP
speaker receives an UPDATE message from a peer located in a speaker receives from an external peer an UPDATE message that adver-
neighboring autonomous system that advertises a new route, a tises a new route, a replacement route, or a withdrawn route.
replacement route, or a withdrawn route.
The Phase 1 decision function is a separate process which completes The Phase 1 decision function is a separate process which completes
when it has no further work to do. when it has no further work to do.
The Phase 1 decision function shall lock an Adj-RIB-In prior to The Phase 1 decision function shall lock an Adj-RIB-In prior to oper-
operating on any route contained within it, and shall unlock it after ating on any route contained within it, and shall unlock it after
operating on all new or unfeasible routes contained within it. operating on all new or unfeasible routes contained within it.
For each newly received or replacement feasible route, the local BGP For each newly received or replacement feasible route, the local BGP
speaker shall determine a degree of preference. If the route is speaker shall determine a degree of preference. If the route is
learned from a BGP speaker in the local autonomous system, either the learned from an internal peer, the value of the LOCAL_PREF attribute
value of the LOCAL_PREF attribute shall be taken as the degree of shall be taken as the degree of preference. If the route is learned
preference, or the local system shall compute the degree of from an external peer, then the degree of preference shall be com-
preference of the route based on preconfigured policy information. If puted based on preconfigured policy information and used as the
the route is learned from a BGP speaker in a neighboring autonomous LOCAL_PREF value in any IBGP readvertisement. The exact nature of
system, then the degree of preference shall be computed based on this policy information and the computation involved is a local mat-
preconfigured policy information. The exact nature of this policy ter. The local speaker shall then run the internal update process of
information and the computation involved is a local matter. The 9.2.1 to select and advertise the most preferable route.
local speaker shall then run the internal update process of 9.2.1 to
select and advertise the most preferable route.
9.1.2 Phase 2: Route Selection 9.1.2 Phase 2: Route Selection
The Phase 2 decision function shall be invoked on completion of Phase The Phase 2 decision function shall be invoked on completion of Phase
1. The Phase 2 function is a separate process which completes when 1. The Phase 2 function is a separate process which completes when
it has no further work to do. The Phase 2 process shall consider all it has no further work to do. The Phase 2 process shall consider all
routes that are present in the Adj-RIBs-In, including those received routes that are present in the Adj-RIBs-In, including those received
from BGP speakers located in its own autonomous system and those from both internal and external peers.
received from BGP speakers located in neighboring autonomous systems.
The Phase 2 decision function shall be blocked from running while the The Phase 2 decision function shall be blocked from running while the
Phase 3 decision function is in process. The Phase 2 function shall Phase 3 decision function is in process. The Phase 2 function shall
lock all Adj-RIBs-In prior to commencing its function, and shall lock all Adj-RIBs-In prior to commencing its function, and shall
unlock them on completion. unlock them on completion.
If the NEXT_HOP attribute of a BGP route depicts an address to which If the NEXT_HOP attribute of a BGP route depicts an address to which
the local BGP speaker doesn't have a route in its Loc-RIB, the BGP the local BGP speaker doesn't have a route in its Loc-RIB, the BGP
route SHOULD be excluded from the Phase 2 decision function. route should be excluded from the Phase 2 decision function.
It is critical that routers within an AS do not make conflicting
decisions regarding route selection that would cause forwarding loops
to occur.
For each set of destinations for which a feasible route exists in the For each set of destinations for which a feasible route exists in the
Adj-RIBs-In, the local BGP speaker shall identify the route that has: Adj-RIBs-In, the local BGP speaker shall identify the route that has:
a) the highest degree of preference of any route to the same set a) the highest degree of preference of any route to the same set
of destinations, or of destinations, or
b) is the only route to that destination, or b) is the only route to that destination, or
c) is selected as a result of the Phase 2 tie breaking rules c) is selected as a result of the Phase 2 tie breaking rules spec-
specified in 9.1.2.1. ified in 9.1.2.1.
The local speaker SHALL then install that route in the Loc-RIB, The local speaker SHALL then install that route in the Loc-RIB,
replacing any route to the same destination that is currently being replacing any route to the same destination that is currently being
held in the Loc-RIB. The local speaker MUST determine the immediate held in the Loc-RIB. The local speaker MUST determine the immediate
next hop to the address depicted by the NEXT_HOP attribute of the next hop to the address depicted by the NEXT_HOP attribute of the
selected route by performing a lookup in the IGP and selecting one of selected route by performing a lookup in the IGP and selecting one of
the possible paths in the IGP. This immediate next hop MUST be used the possible paths in the IGP. This immediate next hop MUST be used
when installing the selected route in the Loc-RIB. If the route to when installing the selected route in the Loc-RIB. If the route to
the address depicted by the NEXT_HOP attribute changes such that the the address depicted by the NEXT_HOP attribute changes such that the
immediate next hop changes, route selection should be recalculated as immediate next hop changes, route selection should be recalculated as
specified above. specified above.
Unfeasible routes shall be removed from the Loc-RIB, and Unfeasible routes shall be removed from the Loc-RIB, and correspond-
corresponding unfeasible routes shall then be removed from the Adj- ing unfeasible routes shall then be removed from the Adj-RIBs-In.
RIBs-In.
9.1.2.1 Breaking Ties (Phase 2) 9.1.2.1 Breaking Ties (Phase 2)
In its Adj-RIBs-In a BGP speaker may have several routes to the same In its Adj-RIBs-In a BGP speaker may have several routes to the same
destination that have the same degree of preference. The local destination that have the same degree of preference. The local
speaker can select only one of these routes for inclusion in the speaker can select only one of these routes for inclusion in the
associated Loc-RIB. The local speaker considers all equally associated Loc-RIB. The local speaker considers all routes with the
preferable routes, both those received from BGP speakers located in same degress of preference, both those received from internal peers,
neighboring autonomous systems, and those received from other BGP and those received from external peers.
speakers located in the local speaker's autonomous system.
The following tie-breaking procedure assumes that for each candidate The following tie-breaking procedure assumes that for each candidate
route all the BGP speakers within an autonomous system can ascertain route all the BGP speakers within an autonomous system can ascertain
the cost of a path (interior distance) to the address depicted by the the cost of a path (interior distance) to the address depicted by the
NEXT_HOP attribute of the route. Ties shall be broken according to NEXT_HOP attribute of the route.
the following algorithm:
a) If the local system is configured to take into account The tie-breaking algorithm begins by considering all equally prefer-
MULTI_EXIT_DISC, and the candidate routes differ in their able routes and then selects routes to be removed from consideration.
MULTI_EXIT_DISC attribute, select the route that has the lowest The algorithm terminates as soon as only one route remains in consid-
value of the MULTI_EXIT_DISC attribute. A route with eration. The criteria must be applied in the order specified.
MULTI_EXIT_DISC shall be preferred to a route without
MULTI_EXIT_DIST.
b) Otherwise, select the route that has the lowest cost (interior Several of the criteria are described using pseudo-code. Note that
distance) to the entity depicted by the NEXT_HOP attribute of the the pseudo-code shown was chosen for clarity, not efficiency. It is
route. If there are several routes with the same cost, then the not intended to specify any particular implementation. BGP implemen-
tie-breaking shall be broken as follows: tations MAY use any algorithm which produces the same results as
those described here.
- if at least one of the candidate routes was advertised by the a) Remove from consideration routes with less-preferred
BGP speaker in a neighboring autonomous system, select the MULTI_EXIT_DISC attributes. MULTI_EXIT_DISC is only comparable
route that was advertised by the BGP speaker in a neighboring between routes learned from the same neighboring AS. Routes which
autonomous system whose BGP Identifier has the lowest value do not have the MULTI_EXIT_DISC attribute are considered to have
among all other BGP speakers in neighboring autonomous systems; the highest possible MULTI_EXIT_DISC value.
- otherwise, select the route that was advertised by the BGP This is also described in the following procedure:
speaker whose BGP Identifier has the lowest value.
for m = all routes still under consideration
for n = all routes still under consideration
if (neighborAS(m) == neighborAS(n)) and (MED(n) < MED(m))
remove route m from consideration
In the pseudo-code above, MED(n) is a function which returns the
value of route n's MULTI_EXIT_DISC attribute. If route n has no
MULTI_EXIT_DISC attribute, the function returns the highest possi-
ble MULTI_EXIT_DISC value, i.e. 2^32-1.
Similarly, neighborAS(n) is a function which returns the neighbor
AS from which the route was received.
b) Remove from consideration any routes with less-preferred inte-
rior cost. The interior cost of a route is determined by calcu-
lating the metric to the next hop for the route using the interior
routing protocol(s). If the next hop for a route is reachable,
but no cost can be determined, then this step should be should be
skipped (equivalently, consider all routes to have equal costs).
This is also described in the following procedure.
for m = all routes still under consideration
for n = all routes in still under consideration
if (cost(n) is better than cost(m))
remove m from consideration
In the pseudo-code above, cost(n) is a function which returns the
cost of the path (interior distance) to the address given in the
NEXT_HOP attribute of the route.
c) If at least one of the candidate routes was received from an
external peer in a neighboring autonomous system, remove from con-
sideration all routes which were received from internal peers.
d) Remove from consideration all routes other than the route that
was advertised by the BGP speaker whose BGP Identifier has the
lowest value.
9.1.3 Phase 3: Route Dissemination 9.1.3 Phase 3: Route Dissemination
The Phase 3 decision function shall be invoked on completion of Phase The Phase 3 decision function shall be invoked on completion of Phase
2, or when any of the following events occur: 2, or when any of the following events occur:
a) when routes in a Loc-RIB to local destinations have changed a) when routes in a Loc-RIB to local destinations have changed
b) when locally generated routes learned by means outside of BGP b) when locally generated routes learned by means outside of BGP
have changed have changed
c) when a new BGP speaker - BGP speaker connection has been c) when a new BGP speaker - BGP speaker connection has been estab-
established lished
The Phase 3 function is a separate process which completes when it The Phase 3 function is a separate process which completes when it
has no further work to do. The Phase 3 Routing Decision function has no further work to do. The Phase 3 Routing Decision function
shall be blocked from running while the Phase 2 decision function is shall be blocked from running while the Phase 2 decision function is
in process. in process.
All routes in the Loc-RIB shall be processed into a corresponding All routes in the Loc-RIB shall be processed into a corresponding
entry in the associated Adj-RIBs-Out. Route aggregation and entry in the associated Adj-RIBs-Out. Route aggregation and informa-
information reduction techniques (see 9.2.4.1) may optionally be tion reduction techniques (see 9.2.4.1) may optionally be applied.
applied.
For the benefit of future support of inter-AS multicast capabilities, For the benefit of future support of inter-AS multicast capabilities,
a BGP speaker that participates in inter-AS multicast routing shall a BGP speaker that participates in inter-AS multicast routing shall
advertise a route it receives from one of its external peers and if advertise a route it receives from one of its external peers and if
it installs it in its Loc-RIB, it shall advertise it back to the peer it installs it in its Loc-RIB, it shall advertise it back to the peer
from which the route was received. For a BGP speaker that does not from which the route was received. For a BGP speaker that does not
participate in inter-AS multicast routing such an advertisement is participate in inter-AS multicast routing such an advertisement is
optional. When doing such an advertisement, the NEXT_HOP attribute optional. When doing such an advertisement, the NEXT_HOP attribute
should be set to the address of the peer. An implementation may also should be set to the address of the peer. An implementation may also
optimize such an advertisement by truncating information in the optimize such an advertisement by truncating information in the
skipping to change at page 40, line 46 skipping to change at page 42, line 9
such an advertisement. such an advertisement.
When the updating of the Adj-RIBs-Out and the Forwarding Information When the updating of the Adj-RIBs-Out and the Forwarding Information
Base (FIB) is complete, the local BGP speaker shall run the external Base (FIB) is complete, the local BGP speaker shall run the external
update process of 9.2.2. update process of 9.2.2.
9.1.4 Overlapping Routes 9.1.4 Overlapping Routes
A BGP speaker may transmit routes with overlapping Network Layer A BGP speaker may transmit routes with overlapping Network Layer
Reachability Information (NLRI) to another BGP speaker. NLRI overlap Reachability Information (NLRI) to another BGP speaker. NLRI overlap
occurs when a set of destinations are identified in non-matching occurs when a set of destinations are identified in non-matching mul-
multiple routes. Since BGP encodes NLRI using IP prefixes, overlap tiple routes. Since BGP encodes NLRI using IP prefixes, overlap will
will always exhibit subset relationships. A route describing a always exhibit subset relationships. A route describing a smaller
smaller set of destinations (a longer prefix) is said to be more set of destinations (a longer prefix) is said to be more specific
specific than a route describing a larger set of destinations (a than a route describing a larger set of destinations (a shorted pre-
shorted prefix); similarly, a route describing a larger set of fix); similarly, a route describing a larger set of destinations (a
destinations (a shorter prefix) is said to be less specific than a shorter prefix) is said to be less specific than a route describing a
route describing a smaller set of destinations (a longer prefix). smaller set of destinations (a longer prefix).
The precedence relationship effectively decomposes less specific The precedence relationship effectively decomposes less specific
routes into two parts: routes into two parts:
- a set of destinations described only by the less specific - a set of destinations described only by the less specific
route, and route, and
- a set of destinations described by the overlap of the less - a set of destinations described by the overlap of the less spe-
specific and the more specific routes cific and the more specific routes
When overlapping routes are present in the same Adj-RIB-In, the more When overlapping routes are present in the same Adj-RIB-In, the more
specific route shall take precedence, in order from more specific to specific route shall take precedence, in order from more specific to
least specific. least specific.
The set of destinations described by the overlap represents a portion The set of destinations described by the overlap represents a portion
of the less specific route that is feasible, but is not currently in of the less specific route that is feasible, but is not currently in
use. If a more specific route is later withdrawn, the set of use. If a more specific route is later withdrawn, the set of desti-
destinations described by the overlap will still be reachable using nations described by the overlap will still be reachable using the
the less specific route. less specific route.
If a BGP speaker receives overlapping routes, the Decision Process If a BGP speaker receives overlapping routes, the Decision Process
shall take into account the semantics of the overlapping routes. In shall take into account the semantics of the overlapping routes. In
particular, if a BGP speaker accepts the less specific route while particular, if a BGP speaker accepts the less specific route while
rejecting the more specific route from the same peer, then the rejecting the more specific route from the same peer, then the desti-
destinations represented by the overlap may not forward along the ASs nations represented by the overlap may not forward along the ASs
listed in the AS_PATH attribute of that route. Therefore, a BGP listed in the AS_PATH attribute of that route. Therefore, a BGP
speaker has the following choices: speaker has the following choices:
a) Install both the less and the more specific routes a) Install both the less and the more specific routes
b) Install the more specific route only b) Install the more specific route only
c) Install the non-overlapping part of the less specific c) Install the non-overlapping part of the less specific
route only (that implies de-aggregation) route only (that implies de-aggregation)
d) Aggregate the two routes and install the aggregated route d) Aggregate the two routes and install the aggregated route
e) Install the less specific route only e) Install the less specific route only
f) Install neither route f) Install neither route
If a BGP speaker chooses e), then it should add ATOMIC_AGGREGATE If a BGP speaker chooses e), then it MUST add ATOMIC_AGGREGATE
attribute to the route. A route that carries ATOMIC_AGGREGATE attribute to the route. A route that carries ATOMIC_AGGREGATE
attribute can not be de-aggregated. That is, the NLRI of this route attribute can not be de-aggregated. That is, the NLRI of this route
can not be made more specific. Forwarding along such a route does can not be made more specific. Forwarding along such a route does
not guarantee that IP packets will actually traverse only ASs listed not guarantee that IP packets will actually traverse only ASs listed
in the AS_PATH attribute of the route. If a BGP speaker chooses a), in the AS_PATH attribute of the route. If a BGP speaker chooses a),
it must not advertise the more general route without the more it must not advertise the more general route without the more spe-
specific route. cific route.
9.2 Update-Send Process 9.2 Update-Send Process
The Update-Send process is responsible for advertising UPDATE The Update-Send process is responsible for advertising UPDATE mes-
messages to all peers. For example, it distributes the routes chosen sages to all peers. For example, it distributes the routes chosen by
by the Decision Process to other BGP speakers which may be located in the Decision Process to other BGP speakers which may be located in
either the same autonomous system or a neighboring autonomous system. either the same autonomous system or a neighboring autonomous system.
Rules for information exchange between BGP speakers located in Rules for information exchange between BGP speakers located in dif-
different autonomous systems are given in 9.2.2; rules for ferent autonomous systems are given in 9.2.2; rules for information
information exchange between BGP speakers located in the same exchange between BGP speakers located in the same autonomous system
autonomous system are given in 9.2.1. are given in 9.2.1.
Distribution of routing information between a set of BGP speakers, Distribution of routing information between a set of BGP speakers,
all of which are located in the same autonomous system, is referred all of which are located in the same autonomous system, is referred
to as internal distribution. to as internal distribution.
9.2.1 Internal Updates 9.2.1 Internal Updates
The Internal update process is concerned with the distribution of The Internal update process is concerned with the distribution of
routing information to BGP speakers located in the local speaker's routing information to internal peers.
autonomous system.
When a BGP speaker receives an UPDATE message from another BGP When a BGP speaker receives an UPDATE message from an internal peer,
speaker located in its own autonomous system, the receiving BGP the receiving BGP speaker shall not re-distribute the routing infor-
speaker shall not re-distribute the routing information contained in mation contained in that UPDATE message to other internal peers.
that UPDATE message to other BGP speakers located in its own
autonomous system.
When a BGP speaker receives a new route from a BGP speaker in a When a BGP speaker receives a new route from an external peer, it
neighboring autonomous system, it shall advertise that route to all shall advertise that route to all other internal peers by means of an
other BGP speakers in its autonomous system by means of an UPDATE UPDATE message if any of the following conditions occur:
message if any of the following conditions occur:
1) the degree of preference assigned to the newly received route 1) the degree of preference assigned to the newly received route
by the local BGP speaker is higher than the degree of preference by the local BGP speaker is higher than the degree of preference
that the local speaker has assigned to other routes that have been that the local speaker has assigned to other routes that have been
received from BGP speakers in neighboring autonomous systems, or received from external peers, or
2) there are no other routes that have been received from BGP 2) there are no other routes that have been received from external
speakers in neighboring autonomous systems, or peers, or
3) the newly received route is selected as a result of breaking a 3) the newly received route is selected as a result of breaking a
tie between several routes which have the highest degree of tie between several routes which have the highest degree of pref-
preference, and the same destination (the tie-breaking procedure erence, and the same destination (the tie-breaking procedure is
is specified in 9.2.1.1). specified in 9.2.1.1).
When a BGP speaker receives an UPDATE message with a non-empty When a BGP speaker receives an UPDATE message with a non-empty WITH-
WITHDRAWN ROUTES field, it shall remove from its Adj-RIB-In all DRAWN ROUTES field, it shall remove from its Adj-RIB-In all routes
routes whose destinations was carried in this field (as IP prefixes). whose destinations was carried in this field (as IP prefixes). The
The speaker shall take the following additional steps: speaker shall take the following additional steps:
1) if the corresponding feasible route had not been previously 1) if the corresponding feasible route had not been previously
advertised, then no further action is necessary advertised, then no further action is necessary
2) if the corresponding feasible route had been previously 2) if the corresponding feasible route had been previously adver-
advertised, then: tised, then:
i) if a new route is selected for advertisement that has the i) if a new route is selected for advertisement that has the
same Network Layer Reachability Information as the unfeasible same Network Layer Reachability Information as the unfeasible
routes, then the local BGP speaker shall advertise the routes, then the local BGP speaker shall advertise the replace-
replacement route ment route
ii) if a replacement route is not available for advertisement, ii) if a replacement route is not available for advertisement,
then the BGP speaker shall include the destinations of the then the BGP speaker shall include the destinations of the
unfeasible route (in form of IP prefixes) in the WITHDRAWN unfeasible route (in form of IP prefixes) in the WITHDRAWN
ROUTES field of an UPDATE message, and shall send this message ROUTES field of an UPDATE message, and shall send this message
to each peer to whom it had previously advertised the to each peer to whom it had previously advertised the corre-
corresponding feasible route. sponding feasible route.
All feasible routes which are advertised shall be placed in the All feasible routes which are advertised shall be placed in the
appropriate Adj-RIBs-Out, and all unfeasible routes which are appropriate Adj-RIBs-Out, and all unfeasible routes which are adver-
advertised shall be removed from the Adj-RIBs-Out. tised shall be removed from the Adj-RIBs-Out.
9.2.1.1 Breaking Ties (Internal Updates) 9.2.1.1 Breaking Ties (Internal Updates)
If a local BGP speaker has connections to several BGP speakers in If a local BGP speaker has connections to several external peers,
neighboring autonomous systems, there will be multiple Adj-RIBs-In there will be multiple Adj-RIBs-In associated with these peers. These
associated with these peers. These Adj-RIBs-In might contain several Adj-RIBs-In might contain several equally preferable routes to the
equally preferable routes to the same destination, all of which were same destination, all of which were advertised by external peers.
advertised by BGP speakers located in neighboring autonomous systems.
The local BGP speaker shall select one of these routes according to The local BGP speaker shall select one of these routes according to
the following rules: the following rules:
a) If the candidate routes differ only in their NEXT_HOP and a) If the candidate routes differ only in their NEXT_HOP and
MULTI_EXIT_DISC attributes, and the local system is configured to MULTI_EXIT_DISC attributes, and the local system is configured to
take into account the MULTI_EXIT_DISC attribute, select the route take into account the MULTI_EXIT_DISC attribute, select the route
that has the lowest value of the MULTI_EXIT_DISC attribute. A that has the lowest value of the MULTI_EXIT_DISC attribute. A
route with the MULTI_EXIT_DISC attribute shall be preferred to a route with the MULTI_EXIT_DISC attribute shall be preferred to a
route without the MULTI_EXIT_DISC attribute. route without the MULTI_EXIT_DISC attribute.
b) If the local system can ascertain the cost of a path to the b) If the local system can ascertain the cost of a path to the
entity depicted by the NEXT_HOP attribute of the candidate route, entity depicted by the NEXT_HOP attribute of the candidate route,
select the route with the lowest cost. select the route with the lowest cost.
c) In all other cases, select the route that was advertised by the c) In all other cases, select the route that was advertised by the
BGP speaker whose BGP Identifier has the lowest value. BGP speaker whose BGP Identifier has the lowest value.
9.2.2 External Updates 9.2.2 External Updates
The external update process is concerned with the distribution of The external update process is concerned with the distribution of
routing information to BGP speakers located in neighboring autonomous routing information to external peers. As part of Phase 3 route
systems. As part of Phase 3 route selection process, the BGP speaker selection process, the BGP speaker has updated its Adj-RIBs-Out and
has updated its Adj-RIBs-Out and its Forwarding Table. All newly its Forwarding Table. All newly installed routes and all newly unfea-
installed routes and all newly unfeasible routes for which there is sible routes for which there is no replacement route shall be adver-
no replacement route shall be advertised to BGP speakers located in tised to external peers by means of UPDATE message.
neighboring autonomous systems by means of UPDATE message.
Any routes in the Loc-RIB marked as unfeasible shall be removed. Any routes in the Loc-RIB marked as unfeasible shall be removed.
Changes to the reachable destinations within its own autonomous Changes to the reachable destinations within its own autonomous sys-
system shall also be advertised in an UPDATE message. tem shall also be advertised in an UPDATE message.
9.2.3 Controlling Routing Traffic Overhead 9.2.3 Controlling Routing Traffic Overhead
The BGP protocol constrains the amount of routing traffic (that is, The BGP protocol constrains the amount of routing traffic (that is,
UPDATE messages) in order to limit both the link bandwidth needed to UPDATE messages) in order to limit both the link bandwidth needed to
advertise UPDATE messages and the processing power needed by the advertise UPDATE messages and the processing power needed by the
Decision Process to digest the information contained in the UPDATE Decision Process to digest the information contained in the UPDATE
messages. messages.
9.2.3.1 Frequency of Route Advertisement 9.2.3.1 Frequency of Route Advertisement
The parameter MinRouteAdvertisementInterval determines the minimum The parameter MinRouteAdvertisementInterval determines the minimum
amount of time that must elapse between advertisement of routes to a amount of time that must elapse between advertisement of routes to a
particular destination from a single BGP speaker. This rate limiting particular destination from a single BGP speaker. This rate limiting
procedure applies on a per-destination basis, although the value of procedure applies on a per-destination basis, although the value of
MinRouteAdvertisementInterval is set on a per BGP peer basis. MinRouteAdvertisementInterval is set on a per BGP peer basis.
Two UPDATE messages sent from a single BGP speaker that advertise Two UPDATE messages sent from a single BGP speaker that advertise
feasible routes to some common set of destinations received from BGP feasible routes to some common set of destinations received from
speakers in neighboring autonomous systems must be separated by at external peers must be separated by at least MinRouteAdvertisementIn-
least MinRouteAdvertisementInterval. Clearly, this can only be terval. Clearly, this can only be achieved precisely by keeping a
achieved precisely by keeping a separate timer for each common set of separate timer for each common set of destinations. This would be
destinations. This would be unwarranted overhead. Any technique which unwarranted overhead. Any technique which ensures that the interval
ensures that the interval between two UPDATE messages sent from a between two UPDATE messages sent from a single BGP speaker that
single BGP speaker that advertise feasible routes to some common set advertise feasible routes to some common set of destinations received
of destinations received from BGP speakers in neighboring autonomous from external peers will be at least MinRouteAdvertisementInterval,
systems will be at least MinRouteAdvertisementInterval, and will also and will also ensure a constant upper bound on the interval is
ensure a constant upper bound on the interval is acceptable. acceptable.
Since fast convergence is needed within an autonomous system, this Since fast convergence is needed within an autonomous system, this
procedure does not apply for routes receives from other BGP speakers procedure does not apply for routes receives from other internal
in the same autonomous system. To avoid long-lived black holes, the peers. To avoid long-lived black holes, the procedure does not apply
procedure does not apply to the explicit withdrawal of unfeasible to the explicit withdrawal of unfeasible routes (that is, routes
routes (that is, routes whose destinations (expressed as IP prefixes) whose destinations (expressed as IP prefixes) are listed in the WITH-
are listed in the WITHDRAWN ROUTES field of an UPDATE message). DRAWN ROUTES field of an UPDATE message).
This procedure does not limit the rate of route selection, but only This procedure does not limit the rate of route selection, but only
the rate of route advertisement. If new routes are selected multiple the rate of route advertisement. If new routes are selected multiple
times while awaiting the expiration of MinRouteAdvertisementInterval, times while awaiting the expiration of MinRouteAdvertisementInterval,
the last route selected shall be advertised at the end of the last route selected shall be advertised at the end of MinRouteAd-
MinRouteAdvertisementInterval. vertisementInterval.
9.2.3.2 Frequency of Route Origination 9.2.3.2 Frequency of Route Origination
The parameter MinASOriginationInterval determines the minimum amount The parameter MinASOriginationInterval determines the minimum amount
of time that must elapse between successive advertisements of UPDATE of time that must elapse between successive advertisements of UPDATE
messages that report changes within the advertising BGP speaker's own messages that report changes within the advertising BGP speaker's own
autonomous systems. autonomous systems.
9.2.3.3 Jitter 9.2.3.3 Jitter
To minimize the likelihood that the distribution of BGP messages by a To minimize the likelihood that the distribution of BGP messages by a
given BGP speaker will contain peaks, jitter should be applied to the given BGP speaker will contain peaks, jitter should be applied to the
timers associated with MinASOriginationInterval, Keepalive, and timers associated with MinASOriginationInterval, Keepalive, and Min-
MinRouteAdvertisementInterval. A given BGP speaker shall apply the RouteAdvertisementInterval. A given BGP speaker shall apply the same
same jitter to each of these quantities regardless of the jitter to each of these quantities regardless of the destinations to
destinations to which the updates are being sent; that is, jitter which the updates are being sent; that is, jitter will not be applied
will not be applied on a "per peer" basis. on a "per peer" basis.
The amount of jitter to be introduced shall be determined by The amount of jitter to be introduced shall be determined by multi-
multiplying the base value of the appropriate timer by a random plying the base value of the appropriate timer by a random factor
factor which is uniformly distributed in the range from 0.75 to 1.0. which is uniformly distributed in the range from 0.75 to 1.0.
9.2.4 Efficient Organization of Routing Information 9.2.4 Efficient Organization of Routing Information
Having selected the routing information which it will advertise, a Having selected the routing information which it will advertise, a
BGP speaker may avail itself of several methods to organize this BGP speaker may avail itself of several methods to organize this
information in an efficient manner. information in an efficient manner.
9.2.4.1 Information Reduction 9.2.4.1 Information Reduction
Information reduction may imply a reduction in granularity of policy Information reduction may imply a reduction in granularity of policy
control - after information is collapsed, the same policies will control - after information is collapsed, the same policies will
apply to all destinations and paths in the equivalence class. apply to all destinations and paths in the equivalence class.
The Decision Process may optionally reduce the amount of information The Decision Process may optionally reduce the amount of information
that it will place in the Adj-RIBs-Out by any of the following that it will place in the Adj-RIBs-Out by any of the following meth-
methods: ods:
a) Network Layer Reachability Information (NLRI): a) Network Layer Reachability Information (NLRI):
Destination IP addresses can be represented as IP address Destination IP addresses can be represented as IP address pre-
prefixes. In cases where there is a correspondence between the fixes. In cases where there is a correspondence between the
address structure and the systems under control of an autonomous address structure and the systems under control of an autonomous
system administrator, it will be possible to reduce the size of system administrator, it will be possible to reduce the size of
the NLRI carried in the UPDATE messages. the NLRI carried in the UPDATE messages.
b) AS_PATHs: b) AS_PATHs:
AS path information can be represented as ordered AS_SEQUENCEs or AS path information can be represented as ordered AS_SEQUENCEs or
unordered AS_SETs. AS_SETs are used in the route aggregation unordered AS_SETs. AS_SETs are used in the route aggregation algo-
algorithm described in 9.2.4.2. They reduce the size of the rithm described in 9.2.4.2. They reduce the size of the AS_PATH
AS_PATH information by listing each AS number only once, information by listing each AS number only once, regardless of how
regardless of how many times it may have appeared in multiple many times it may have appeared in multiple AS_PATHs that were
AS_PATHs that were aggregated. aggregated.
An AS_SET implies that the destinations listed in the NLRI can be An AS_SET implies that the destinations listed in the NLRI can be
reached through paths that traverse at least some of the reached through paths that traverse at least some of the con-
constituent autonomous systems. AS_SETs provide sufficient stituent autonomous systems. AS_SETs provide sufficient informa-
information to avoid routing information looping; however their tion to avoid routing information looping; however their use may
use may prune potentially feasible paths, since such paths are no prune potentially feasible paths, since such paths are no longer
longer listed individually as in the form of AS_SEQUENCEs. In listed individually as in the form of AS_SEQUENCEs. In practice
practice this is not likely to be a problem, since once an IP this is not likely to be a problem, since once an IP packet
packet arrives at the edge of a group of autonomous systems, the arrives at the edge of a group of autonomous systems, the BGP
BGP speaker at that point is likely to have more detailed path speaker at that point is likely to have more detailed path infor-
information and can distinguish individual paths to destinations. mation and can distinguish individual paths to destinations.
9.2.4.2 Aggregating Routing Information 9.2.4.2 Aggregating Routing Information
Aggregation is the process of combining the characteristics of Aggregation is the process of combining the characteristics of sev-
several different routes in such a way that a single route can be eral different routes in such a way that a single route can be adver-
advertised. Aggregation can occur as part of the decision process tised. Aggregation can occur as part of the decision process to
to reduce the amount of routing information that will be placed in reduce the amount of routing information that will be placed in the
the Adj-RIBs-Out. Adj-RIBs-Out.
Aggregation reduces the amount of information that a BGP speaker must Aggregation reduces the amount of information that a BGP speaker must
store and exchange with other BGP speakers. Routes can be aggregated store and exchange with other BGP speakers. Routes can be aggregated
by applying the following procedure separately to path attributes of by applying the following procedure separately to path attributes of
like type and to the Network Layer Reachability Information. like type and to the Network Layer Reachability Information.
Routes that have the following attributes shall not be aggregated Routes that have the following attributes shall not be aggregated
unless the corresponding attributes of each route are identical: unless the corresponding attributes of each route are identical:
MULTI_EXIT_DISC, NEXT_HOP. MULTI_EXIT_DISC, NEXT_HOP.
Path attributes that have different type codes can not be aggregated Path attributes that have different type codes can not be aggregated
together. Path of the same type code may be aggregated, according to together. Path of the same type code may be aggregated, according to
the following rules: the following rules:
ORIGIN attribute: If at least one route among routes that are ORIGIN attribute: If at least one route among routes that are
aggregated has ORIGIN with the value INCOMPLETE, then the aggregated has ORIGIN with the value INCOMPLETE, then the aggre-
aggregated route must have the ORIGIN attribute with the value gated route must have the ORIGIN attribute with the value INCOM-
INCOMPLETE. Otherwise, if at least one route among routes that are PLETE. Otherwise, if at least one route among routes that are
aggregated has ORIGIN with the value EGP, then the aggregated aggregated has ORIGIN with the value EGP, then the aggregated
route must have the origin attribute with the value EGP. In all route must have the origin attribute with the value EGP. In all
other case the value of the ORIGIN attribute of the aggregated other case the value of the ORIGIN attribute of the aggregated
route is INTERNAL. route is INTERNAL.
AS_PATH attribute: If routes to be aggregated have identical AS_PATH attribute: If routes to be aggregated have identical
AS_PATH attributes, then the aggregated route has the same AS_PATH AS_PATH attributes, then the aggregated route has the same AS_PATH
attribute as each individual route. attribute as each individual route.
For the purpose of aggregating AS_PATH attributes we model each AS For the purpose of aggregating AS_PATH attributes we model each AS
within the AS_PATH attribute as a tuple <type, value>, where within the AS_PATH attribute as a tuple <type, value>, where
"type" identifies a type of the path segment the AS belongs to "type" identifies a type of the path segment the AS belongs to
(e.g. AS_SEQUENCE, AS_SET), and "value" is the AS number. If the (e.g. AS_SEQUENCE, AS_SET), and "value" is the AS number. If the
routes to be aggregated have different AS_PATH attributes, then routes to be aggregated have different AS_PATH attributes, then
the aggregated AS_PATH attribute shall satisfy all of the the aggregated AS_PATH attribute shall satisfy all of the follow-
following conditions: ing conditions:
- all tuples of the type AS_SEQUENCE in the aggregated AS_PATH - all tuples of the type AS_SEQUENCE in the aggregated AS_PATH
shall appear in all of the AS_PATH in the initial set of routes shall appear in all of the AS_PATH in the initial set of routes
to be aggregated. to be aggregated.
- all tuples of the type AS_SET in the aggregated AS_PATH shall - all tuples of the type AS_SET in the aggregated AS_PATH shall
appear in at least one of the AS_PATH in the initial set (they appear in at least one of the AS_PATH in the initial set (they
may appear as either AS_SET or AS_SEQUENCE types). may appear as either AS_SET or AS_SEQUENCE types).
- for any tuple X of the type AS_SEQUENCE in the aggregated - for any tuple X of the type AS_SEQUENCE in the aggregated
AS_PATH which precedes tuple Y in the aggregated AS_PATH, X AS_PATH which precedes tuple Y in the aggregated AS_PATH, X
precedes Y in each AS_PATH in the initial set which contains Y, precedes Y in each AS_PATH in the initial set which contains Y,
regardless of the type of Y. regardless of the type of Y.
- No tuple with the same value shall appear more than once in - No tuple with the same value shall appear more than once in
the aggregated AS_PATH, regardless of the tuple's type. the aggregated AS_PATH, regardless of the tuple's type.
An implementation may choose any algorithm which conforms to these An implementation may choose any algorithm which conforms to these
rules. At a minimum a conformant implementation shall be able to rules. At a minimum a conformant implementation shall be able to
perform the following algorithm that meets all of the above perform the following algorithm that meets all of the above condi-
conditions: tions:
- determine the longest leading sequence of tuples (as defined - determine the longest leading sequence of tuples (as defined
above) common to all the AS_PATH attributes of the routes to be above) common to all the AS_PATH attributes of the routes to be
aggregated. Make this sequence the leading sequence of the aggregated. Make this sequence the leading sequence of the
aggregated AS_PATH attribute. aggregated AS_PATH attribute.
- set the type of the rest of the tuples from the AS_PATH - set the type of the rest of the tuples from the AS_PATH
attributes of the routes to be aggregated to AS_SET, and append attributes of the routes to be aggregated to AS_SET, and append
them to the aggregated AS_PATH attribute. them to the aggregated AS_PATH attribute.
skipping to change at page 48, line 47 skipping to change at page 49, line 49
such tuple by deleting tuples of the type AS_SET from the such tuple by deleting tuples of the type AS_SET from the
aggregated AS_PATH attribute. aggregated AS_PATH attribute.
Appendix 6, section 6.8 presents another algorithm that satisfies Appendix 6, section 6.8 presents another algorithm that satisfies
the conditions and allows for more complex policy configurations. the conditions and allows for more complex policy configurations.
ATOMIC_AGGREGATE: If at least one of the routes to be aggregated ATOMIC_AGGREGATE: If at least one of the routes to be aggregated
has ATOMIC_AGGREGATE path attribute, then the aggregated route has ATOMIC_AGGREGATE path attribute, then the aggregated route
shall have this attribute as well. shall have this attribute as well.
AGGREGATOR: All AGGREGATOR attributes of all routes to be AGGREGATOR: All AGGREGATOR attributes of all routes to be aggre-
aggregated should be ignored. gated should be ignored.
9.3 Route Selection Criteria 9.3 Route Selection Criteria
Generally speaking, additional rules for comparing routes among Generally speaking, additional rules for comparing routes among sev-
several alternatives are outside the scope of this document. There eral alternatives are outside the scope of this document. There are
are two exceptions: two exceptions:
- If the local AS appears in the AS path of the new route being - If the local AS appears in the AS path of the new route being
considered, then that new route cannot be viewed as better than considered, then that new route cannot be viewed as better than
any other route. If such a route were ever used, a routing loop any other route. If such a route were ever used, a routing loop
would result. would result.
- In order to achieve successful distributed operation, only - In order to achieve successful distributed operation, only
routes with a likelihood of stability can be chosen. Thus, an AS routes with a likelihood of stability can be chosen. Thus, an AS
must avoid using unstable routes, and it must not make rapid must avoid using unstable routes, and it must not make rapid spon-
spontaneous changes to its choice of route. Quantifying the terms taneous changes to its choice of route. Quantifying the terms
"unstable" and "rapid" in the previous sentence will require "unstable" and "rapid" in the previous sentence will require expe-
experience, but the principle is clear. rience, but the principle is clear.
9.4 Originating BGP routes 9.4 Originating BGP routes
A BGP speaker may originate BGP routes by injecting routing A BGP speaker may originate BGP routes by injecting routing informa-
information acquired by some other means (e.g. via an IGP) into BGP. tion acquired by some other means (e.g. via an IGP) into BGP. A BGP
A BGP speaker that originates BGP routes shall assign the degree of speaker that originates BGP routes shall assign the degree of prefer-
preference to these routes by passing them through the Decision ence to these routes by passing them through the Decision Process
Process (see Section 9.1). These routes may also be distributed to (see Section 9.1). These routes may also be distributed to other BGP
other BGP speakers within the local AS as part of the Internal update speakers within the local AS as part of the Internal update process
process (see Section 9.2.1). The decision whether to distribute non- (see Section 9.2.1). The decision whether to distribute non-BGP
BGP acquired routes within an AS via BGP or not depends on the acquired routes within an AS via BGP or not depends on the environ-
environment within the AS (e.g. type of IGP) and should be controlled ment within the AS (e.g. type of IGP) and should be controlled via
via configuration. configuration.
Appendix 1. BGP FSM State Transitions and Actions. Appendix 1. BGP FSM State Transitions and Actions.
This Appendix discusses the transitions between states in the BGP FSM This Appendix discusses the transitions between states in the BGP FSM
in response to BGP events. The following is the list of these states in response to BGP events. The following is the list of these states
and events when the negotiated Hold Time value is non-zero. and events when the negotiated Hold Time value is non-zero.
BGP States: BGP States:
1 - Idle 1 - Idle
skipping to change at page 52, line 39 skipping to change at page 53, line 39
11 | 1 | 1 | 1 | 1 | 6 | 6 11 | 1 | 1 | 1 | 1 | 6 | 6
| | | | | | | | | | | |
12 | 1 | 1 | 1 | 1 | 1 | 1 or 6 12 | 1 | 1 | 1 | 1 | 1 | 1 or 6
| | | | | | | | | | | |
13 | 1 | 1 | 1 | 1 | 1 | 1 13 | 1 | 1 | 1 | 1 | 1 | 1
| | | | | | | | | | | |
--------------------------------------------------------------- ---------------------------------------------------------------
Appendix 2. Comparison with RFC1267 Appendix 2. Comparison with RFC1267
BGP-4 is capable of operating in an environment where a set of BGP-4 is capable of operating in an environment where a set of reach-
reachable destinations may be expressed via a single IP prefix. The able destinations may be expressed via a single IP prefix. The con-
concept of network classes, or subnetting is foreign to BGP-4. To cept of network classes, or subnetting is foreign to BGP-4. To
accommodate these capabilities BGP-4 changes semantics and encoding accommodate these capabilities BGP-4 changes semantics and encoding
associated with the AS_PATH attribute. New text has been added to associated with the AS_PATH attribute. New text has been added to
define semantics associated with IP prefixes. These abilities allow define semantics associated with IP prefixes. These abilities allow
BGP-4 to support the proposed supernetting scheme [9]. BGP-4 to support the proposed supernetting scheme [9].
To simplify configuration this version introduces a new attribute, To simplify configuration this version introduces a new attribute,
LOCAL_PREF, that facilitates route selection procedures. LOCAL_PREF, that facilitates route selection procedures.
The INTER_AS_METRIC attribute has been renamed to be MULTI_EXIT_DISC. The INTER_AS_METRIC attribute has been renamed to be MULTI_EXIT_DISC.
A new attribute, ATOMIC_AGGREGATE, has been introduced to insure that A new attribute, ATOMIC_AGGREGATE, has been introduced to insure that
certain aggregates are not de-aggregated. Another new attribute, certain aggregates are not de-aggregated. Another new attribute,
AGGREGATOR, can be added to aggregate routes in order to advertise AGGREGATOR, can be added to aggregate routes in order to advertise
which AS and which BGP speaker within that AS caused the aggregation. which AS and which BGP speaker within that AS caused the aggregation.
To insure that Hold Timers are symmetric, the Hold Time is now To insure that Hold Timers are symmetric, the Hold Time is now nego-
negotiated on a per-connection basis. Hold Times of zero are now tiated on a per-connection basis. Hold Times of zero are now sup-
supported. ported.
Appendix 3. Comparison with RFC 1163 Appendix 3. Comparison with RFC 1163
All of the changes listed in Appendix 2, plus the following. All of the changes listed in Appendix 2, plus the following.
To detect and recover from BGP connection collision, a new field (BGP To detect and recover from BGP connection collision, a new field (BGP
Identifier) has been added to the OPEN message. New text (Section Identifier) has been added to the OPEN message. New text (Section
6.8) has been added to specify the procedure for detecting and 6.8) has been added to specify the procedure for detecting and recov-
recovering from collision. ering from collision.
The new document no longer restricts the border router that is passed The new document no longer restricts the border router that is passed
in the NEXT_HOP path attribute to be part of the same Autonomous in the NEXT_HOP path attribute to be part of the same Autonomous Sys-
System as the BGP Speaker. tem as the BGP Speaker.
New document optimizes and simplifies the exchange of the information New document optimizes and simplifies the exchange of the information
about previously reachable routes. about previously reachable routes.
Appendix 4. Comparison with RFC 1105 Appendix 4. Comparison with RFC 1105
All of the changes listed in Appendices 2 and 3, plus the following. All of the changes listed in Appendices 2 and 3, plus the following.
Minor changes to the RFC1105 Finite State Machine were necessary to Minor changes to the RFC1105 Finite State Machine were necessary to
accommodate the TCP user interface provided by 4.3 BSD. accommodate the TCP user interface provided by 4.3 BSD.
skipping to change at page 54, line 14 skipping to change at page 55, line 14
added to the OPEN message. added to the OPEN message.
2. The version field has been removed from the BGP header and 2. The version field has been removed from the BGP header and
added to the OPEN message. added to the OPEN message.
3. The Link Type field has been removed from the OPEN message. 3. The Link Type field has been removed from the OPEN message.
4. The OPEN CONFIRM message has been eliminated and replaced with 4. The OPEN CONFIRM message has been eliminated and replaced with
implicit confirmation provided by the KEEPALIVE message. implicit confirmation provided by the KEEPALIVE message.
5. The format of the UPDATE message has been changed 5. The format of the UPDATE message has been changed signifi-
significantly. New fields were added to the UPDATE message to cantly. New fields were added to the UPDATE message to support
support multiple path attributes. multiple path attributes.
6. The Marker field has been expanded and its role broadened to 6. The Marker field has been expanded and its role broadened to
support authentication. support authentication.
Note that quite often BGP, as specified in RFC 1105, is referred Note that quite often BGP, as specified in RFC 1105, is referred
to as BGP-1, BGP, as specified in RFC 1163, is referred to as to as BGP-1, BGP, as specified in RFC 1163, is referred to as
BGP-2, BGP, as specified in RFC1267 is referred to as BGP-3, and BGP-2, BGP, as specified in RFC1267 is referred to as BGP-3, and
BGP, as specified in this document is referred to as BGP-4. BGP, as specified in this document is referred to as BGP-4.
Appendix 5. TCP options that may be used with BGP Appendix 5. TCP options that may be used with BGP
skipping to change at page 55, line 11 skipping to change at page 56, line 11
6.1 Multiple Networks Per Message 6.1 Multiple Networks Per Message
The BGP protocol allows for multiple address prefixes with the same The BGP protocol allows for multiple address prefixes with the same
AS path and next-hop gateway to be specified in one message. Making AS path and next-hop gateway to be specified in one message. Making
use of this capability is highly recommended. With one address prefix use of this capability is highly recommended. With one address prefix
per message there is a substantial increase in overhead in the per message there is a substantial increase in overhead in the
receiver. Not only does the system overhead increase due to the receiver. Not only does the system overhead increase due to the
reception of multiple messages, but the overhead of scanning the reception of multiple messages, but the overhead of scanning the
routing table for updates to BGP peers and other routing protocols routing table for updates to BGP peers and other routing protocols
(and sending the associated messages) is incurred multiple times as (and sending the associated messages) is incurred multiple times as
well. One method of building messages containing many address well. One method of building messages containing many address pre-
prefixes per AS path and gateway from a routing table that is not fixes per AS path and gateway from a routing table that is not orga-
organized per AS path is to build many messages as the routing table nized per AS path is to build many messages as the routing table is
is scanned. As each address prefix is processed, a message for the scanned. As each address prefix is processed, a message for the asso-
associated AS path and gateway is allocated, if it does not exist, ciated AS path and gateway is allocated, if it does not exist, and
and the new address prefix is added to it. If such a message exists, the new address prefix is added to it. If such a message exists, the
the new address prefix is just appended to it. If the message lacks new address prefix is just appended to it. If the message lacks the
the space to hold the new address prefix, it is transmitted, a new space to hold the new address prefix, it is transmitted, a new mes-
message is allocated, and the new address prefix is inserted into the sage is allocated, and the new address prefix is inserted into the
new message. When the entire routing table has been scanned, all new message. When the entire routing table has been scanned, all
allocated messages are sent and their resources released. Maximum allocated messages are sent and their resources released. Maximum
compression is achieved when all the destinations covered by the compression is achieved when all the destinations covered by the
address prefixes share a gateway and common path attributes, making address prefixes share a gateway and common path attributes, making
it possible to send many address prefixes in one 4096-byte message. it possible to send many address prefixes in one 4096-byte message.
When peering with a BGP implementation that does not compress When peering with a BGP implementation that does not compress multi-
multiple address prefixes into one message, it may be necessary to ple address prefixes into one message, it may be necessary to take
take steps to reduce the overhead from the flood of data received steps to reduce the overhead from the flood of data received when a
when a peer is acquired or a significant network topology change peer is acquired or a significant network topology change occurs. One
occurs. One method of doing this is to limit the rate of updates. method of doing this is to limit the rate of updates. This will elim-
This will eliminate the redundant scanning of the routing table to inate the redundant scanning of the routing table to provide flash
provide flash updates for BGP peers and other routing protocols. A updates for BGP peers and other routing protocols. A disadvantage of
disadvantage of this approach is that it increases the propagation this approach is that it increases the propagation latency of routing
latency of routing information. By choosing a minimum flash update information. By choosing a minimum flash update interval that is not
interval that is not much greater than the time it takes to process much greater than the time it takes to process the multiple messages
the multiple messages this latency should be minimized. A better this latency should be minimized. A better method would be to read
method would be to read all received messages before sending updates. all received messages before sending updates.
6.2 Processing Messages on a Stream Protocol 6.2 Processing Messages on a Stream Protocol
BGP uses TCP as a transport mechanism. Due to the stream nature of BGP uses TCP as a transport mechanism. Due to the stream nature of
TCP, all the data for received messages does not necessarily arrive TCP, all the data for received messages does not necessarily arrive
at the same time. This can make it difficult to process the data as at the same time. This can make it difficult to process the data as
messages, especially on systems such as BSD Unix where it is not messages, especially on systems such as BSD Unix where it is not pos-
possible to determine how much data has been received but not yet sible to determine how much data has been received but not yet pro-
processed. cessed.
One method that can be used in this situation is to first try to read One method that can be used in this situation is to first try to read
just the message header. For the KEEPALIVE message type, this is a just the message header. For the KEEPALIVE message type, this is a
complete message; for other message types, the header should first be complete message; for other message types, the header should first be
verified, in particular the total length. If all checks are verified, in particular the total length. If all checks are success-
successful, the specified length, minus the size of the message ful, the specified length, minus the size of the message header is
header is the amount of data left to read. An implementation that the amount of data left to read. An implementation that would "hang"
would "hang" the routing information process while trying to read the routing information process while trying to read from a peer
from a peer could set up a message buffer (4096 bytes) per peer and could set up a message buffer (4096 bytes) per peer and fill it with
fill it with data as available until a complete message has been data as available until a complete message has been received.
received.
6.3 Reducing route flapping 6.3 Reducing route flapping
To avoid excessive route flapping a BGP speaker which needs to To avoid excessive route flapping a BGP speaker which needs to with-
withdraw a destination and send an update about a more specific or draw a destination and send an update about a more specific or less
less specific route shall combine them into the same UPDATE message. specific route SHOULD combine them into the same UPDATE message.
6.4 BGP Timers 6.4 BGP Timers
BGP employs five timers: ConnectRetry, Hold Time, KeepAlive, BGP employs five timers: ConnectRetry, Hold Time, KeepAlive, MinASO-
MinASOriginationInterval, and MinRouteAdvertisementInterval The riginationInterval, and MinRouteAdvertisementInterval The suggested
suggested value for the ConnectRetry timer is 120 seconds. The value for the ConnectRetry timer is 120 seconds. The suggested value
suggested value for the Hold Time is 90 seconds. The suggested value for the Hold Time is 90 seconds. The suggested value for the
for the KeepAlive timer is 30 seconds. The suggested value for the KeepAlive timer is 30 seconds. The suggested value for the MinASO-
MinASOriginationInterval is 15 seconds. The suggested value for the riginationInterval is 15 seconds. The suggested value for the Min-
MinRouteAdvertisementInterval is 30 seconds. RouteAdvertisementInterval is 30 seconds.
An implementation of BGP MUST allow these timers to be configurable. An implementation of BGP MUST allow these timers to be configurable.
6.5 Path attribute ordering 6.5 Path attribute ordering
Implementations which combine update messages as described above in Implementations which combine update messages as described above in
6.1 may prefer to see all path attributes presented in a known order. 6.1 may prefer to see all path attributes presented in a known order.
This permits them to quickly identify sets of attributes from This permits them to quickly identify sets of attributes from differ-
different update messages which are semantically identical. To ent update messages which are semantically identical. To facilitate
facilitate this, it is a useful optimization to order the path this, it is a useful optimization to order the path attributes
attributes according to type code. This optimization is entirely according to type code. This optimization is entirely optional.
optional.
6.6 AS_SET sorting 6.6 AS_SET sorting
Another useful optimization that can be done to simplify this Another useful optimization that can be done to simplify this situa-
situation is to sort the AS numbers found in an AS_SET. This tion is to sort the AS numbers found in an AS_SET. This optimization
optimization is entirely optional. is entirely optional.
6.7 Control over version negotiation 6.7 Control over version negotiation
Since BGP-4 is capable of carrying aggregated routes which cannot be Since BGP-4 is capable of carrying aggregated routes which cannot be
properly represented in BGP-3, an implementation which supports BGP-4 properly represented in BGP-3, an implementation which supports BGP-4
and another BGP version should provide the capability to only speak and another BGP version should provide the capability to only speak
BGP-4 on a per-peer basis. BGP-4 on a per-peer basis.
6.8 Complex AS_PATH aggregation 6.8 Complex AS_PATH aggregation
An implementation which chooses to provide a path aggregation An implementation which chooses to provide a path aggregation algo-
algorithm which retains significant amounts of path information may rithm which retains significant amounts of path information may wish
wish to use the following procedure: to use the following procedure:
For the purpose of aggregating AS_PATH attributes of two routes, For the purpose of aggregating AS_PATH attributes of two routes,
we model each AS as a tuple <type, value>, where "type" identifies we model each AS as a tuple <type, value>, where "type" identifies
a type of the path segment the AS belongs to (e.g. AS_SEQUENCE, a type of the path segment the AS belongs to (e.g. AS_SEQUENCE,
AS_SET), and "value" is the AS number. Two ASs are said to be the AS_SET), and "value" is the AS number. Two ASs are said to be the
same if their corresponding <type, value> tuples are the same. same if their corresponding <type, value> tuples are the same.
The algorithm to aggregate two AS_PATH attributes works as The algorithm to aggregate two AS_PATH attributes works as fol-
follows: lows:
a) Identify the same ASs (as defined above) within each AS_PATH a) Identify the same ASs (as defined above) within each AS_PATH
attribute that are in the same relative order within both attribute that are in the same relative order within both
AS_PATH attributes. Two ASs, X and Y, are said to be in the AS_PATH attributes. Two ASs, X and Y, are said to be in the
same order if either: same order if either:
- X precedes Y in both AS_PATH attributes, or - Y precedes X - X precedes Y in both AS_PATH attributes, or - Y precedes X
in both AS_PATH attributes. in both AS_PATH attributes.
b) The aggregated AS_PATH attribute consists of ASs identified b) The aggregated AS_PATH attribute consists of ASs identified
in (a) in exactly the same order as they appear in the AS_PATH in (a) in exactly the same order as they appear in the AS_PATH
attributes to be aggregated. If two consecutive ASs identified attributes to be aggregated. If two consecutive ASs identified
in (a) do not immediately follow each other in both of the in (a) do not immediately follow each other in both of the
AS_PATH attributes to be aggregated, then the intervening ASs AS_PATH attributes to be aggregated, then the intervening ASs
(ASs that are between the two consecutive ASs that are the (ASs that are between the two consecutive ASs that are the
same) in both attributes are combined into an AS_SET path same) in both attributes are combined into an AS_SET path seg-
segment that consists of the intervening ASs from both AS_PATH ment that consists of the intervening ASs from both AS_PATH
attributes; this segment is then placed in between the two attributes; this segment is then placed in between the two con-
consecutive ASs identified in (a) of the aggregated attribute. secutive ASs identified in (a) of the aggregated attribute. If
If two consecutive ASs identified in (a) immediately follow two consecutive ASs identified in (a) immediately follow each
each other in one attribute, but do not follow in another, then other in one attribute, but do not follow in another, then the
the intervening ASs of the latter are combined into an AS_SET intervening ASs of the latter are combined into an AS_SET path
path segment; this segment is then placed in between the two segment; this segment is then placed in between the two consec-
consecutive ASs identified in (a) of the aggregated attribute. utive ASs identified in (a) of the aggregated attribute.
If as a result of the above procedure a given AS number appears If as a result of the above procedure a given AS number appears
more than once within the aggregated AS_PATH attribute, all, but more than once within the aggregated AS_PATH attribute, all, but
the last instance (rightmost occurrence) of that AS number should the last instance (rightmost occurrence) of that AS number should
be removed from the aggregated AS_PATH attribute. be removed from the aggregated AS_PATH attribute.
References References
[1] Mills, D., "Exterior Gateway Protocol Formal Specification", RFC [1] Mills, D., "Exterior Gateway Protocol Formal Specification", RFC
904, BBN, April 1984. 904, BBN, April 1984.
[2] Rekhter, Y., "EGP and Policy Based Routing in the New NSFNET [2] Rekhter, Y., "EGP and Policy Based Routing in the New NSFNET
Backbone", RFC 1092, T.J. Watson Research Center, February 1989. Backbone", RFC 1092, T.J. Watson Research Center, February 1989.
[3] Braun, H-W., "The NSFNET Routing Architecture", RFC 1093, [3] Braun, H-W., "The NSFNET Routing Architecture", RFC 1093,
MERIT/NSFNET Project, February 1989. MERIT/NSFNET Project, February 1989.
[4] Postel, J., "Transmission Control Protocol - DARPA Internet [4] Postel, J., "Transmission Control Protocol - DARPA Internet Pro-
Program Protocol Specification", RFC 793, DARPA, September 1981. gram Protocol Specification", RFC 793, DARPA, September 1981.
[5] Rekhter, Y., and P. Gross, "Application of the Border Gateway [5] Rekhter, Y., and P. Gross, "Application of the Border Gateway
Protocol in the Internet", T.J. Watson Research Center, IBM Corp., Protocol in the Internet", T.J. Watson Research Center, IBM Corp.,
MCI, Internet Draft. MCI, Internet Draft.
[6] Postel, J., "Internet Protocol - DARPA Internet Program Protocol [6] Postel, J., "Internet Protocol - DARPA Internet Program Protocol
Specification", RFC 791, DARPA, September 1981. Specification", RFC 791, DARPA, September 1981.
[7] "Information Processing Systems - Telecommunications and [7] "Information Processing Systems - Telecommunications and Informa-
Information Exchange between Systems - Protocol for Exchange of tion Exchange between Systems - Protocol for Exchange of Inter-domain
Inter-domain Routeing Information among Intermediate Systems to Routeing Information among Intermediate Systems to Support Forwarding
Support Forwarding of ISO 8473 PDUs", ISO/IEC IS10747, 1993 of ISO 8473 PDUs", ISO/IEC IS10747, 1993
[8] Fuller, V., Li, T., Yu, J., and Varadhan, K., ""Classless Inter- [8] Fuller, V., Li, T., Yu, J., and Varadhan, K., ""Classless Inter-
Domain Routing (CIDR): an Address Assignment and Aggregation Domain Routing (CIDR): an Address Assignment and Aggregation Strat-
Strategy", RFC 1519, BARRNet, cisco, MERIT, OARnet, September 1993 egy", RFC 1519, BARRNet, cisco, MERIT, OARnet, September 1993
[9] Rekhter, Y., Li, T., "An Architecture for IP Address Allocation [9] Rekhter, Y., Li, T., "An Architecture for IP Address Allocation
with CIDR", RFC 1518, T.J. Watson Research Center, cisco, September with CIDR", RFC 1518, T.J. Watson Research Center, cisco, September
1993 1993
Security Considerations Security Considerations
Security issues are not discussed in this document. Security issues are not discussed in this document.
Editors' Addresses Editors' Addresses
skipping to change at page 59, line 22 skipping to change at page 60, line 18
cisco Systems, Inc. cisco Systems, Inc.
170 W. Tasman Dr. 170 W. Tasman Dr.
San Jose, CA 95134 San Jose, CA 95134
email: yakov@cisco.com email: yakov@cisco.com
Tony Li Tony Li
Juniper Networks, Inc. Juniper Networks, Inc.
3260 Jay St. 3260 Jay St.
Santa Clara, CA 95051 Santa Clara, CA 95051
(408) 327-1906 (408) 327-1906
email: tli@jnx.com email: tli@juniper.net
 End of changes. 

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