draft-ietf-idr-as-dedicated-00.txt   rfc2270.txt 
INTERNET-DRAFT John W. Stewart, III / ISI Network Working Group J. Stewart
<draft-ietf-idr-as-dedicated-00.txt> Tony Bates / Cisco Request for Comments: 2270 ISI
Ravi Chandra / Cisco Category: Informational T. Bates
Enke Chen / Cisco R. Chandra
July 1997 E. Chen
Cisco
January 1998
Using a Dedicated AS for Sites Homed to a Single Provider Using a Dedicated AS for Sites Homed to a Single Provider
<draft-ietf-idr-as-dedicated-00.txt>
Status of this Memo Status of this Memo
This document is an Internet-Draft. Internet-Drafts are working docu- This memo provides information for the Internet community. It does
ments of the Internet Engineering Task Force (IETF), its areas, and not specify an Internet standard of any kind. Distribution of this
its working groups. Note that other groups may also distribute work- memo is unlimited.
ing documents as Internet-Drafts.
Internet-Drafts are draft documents valid for a maximum of six Copyright Notice
months. Internet-Drafts may be updated, replaced or obsoleted by
other documents at any time. It is not appropriate to use Internet-
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draft" or "work in progress."
Please check the abstract listing contained in each Internet-Draft Copyright (C) The Internet Society (1998). All Rights Reserved.
directory to learn the current status of this or any other Internet-
Draft.
Abstract Abstract
With the increased growth of the Internet, the number of customers With the increased growth of the Internet, the number of customers
using BGP4 has grown significantly. RFC1930 outlines a set of guide- using BGP4 has grown significantly. RFC1930 outlines a set of
lines for when one needs and should use an AS. However, the customer guidelines for when one needs and should use an AS. However, the
and service provider (ISP) are left with a problem as a result of customer and service provider (ISP) are left with a problem as a
this in that while there is no need for an allocated AS under the result of this in that while there is no need for an allocated AS
guidelines, certain conditions make the use of BGP4 a very pragmatic under the guidelines, certain conditions make the use of BGP4 a very
and perhaps only way to connect a customer homed to a single ISP. pragmatic and perhaps only way to connect a customer homed to a
This paper proposes a solution to this problem in line with recommen- single ISP. This paper proposes a solution to this problem in line
dations set forth in RFC1930. with recommendations set forth in RFC1930.
1. Problems 1. Problems
With the increased growth of the Internet, the number of customers With the increased growth of the Internet, the number of customers
using BGP4 [1],[2] has grown significantly. RFC1930 [4] outlines a using BGP4 [1],[2] has grown significantly. RFC1930 [4] outlines a
set of guidelines for when one needs and should use an AS. However, set of guidelines for when one needs and should use an AS. However,
the customer and service provider (ISP) are left with a problem as a the customer and service provider (ISP) are left with a problem as a
result of this in that while there is no need for an allocated AS result of this in that while there is no need for an allocated AS
under the guidelines, certain conditions make the use of BGP4 a very under the guidelines, certain conditions make the use of BGP4 a very
pragmatic and perhaps only way to connect a customer homed to a sin- pragmatic and perhaps only way to connect a customer homed to a
gle ISP. These conditions are as follows: single ISP. These conditions are as follows:
1) Customers multi-homed to single provider
Consider the scenario outlined in Figure 1 below. 1) Customers multi-homed to single provider
Consider the scenario outlined in Figure 1 below.
+-------+ +-------+ +-------+ +-------+
+----+ | | | +----+ | | |
+------+ | | ISP A +------+ ISP B | +------+ | | ISP A +------+ ISP B |
| Cust.+---+ | | | | | Cust.+---+ | | | |
| X +--------+ | | | | X +--------+ | | |
+------+ ++-----++\ +-------+ +------+ ++-----++\ +-------+
| | \ | | \
| | \ +--------+ | | \ +--------+
++-----++ +-| | ++-----++ +-| |
| Cust. | | ISP C | | Cust. | | ISP C |
| Y | | | | Y | | |
+-------+ +--------+ +-------+ +--------+
Figure 1: Customers multi-home to a single provider Figure 1: Customers multi-home to a single provider
Here both customer X and customer Y are multi-homed to a single Here both customer X and customer Y are multi-homed to a single
provider, ISP A. Because these multiple connections are "local- provider, ISP A. Because these multiple connections are "localized"
ized" between the ISP A and its customers, the rest of the routing between the ISP A and its customers, the rest of the routing system
system (ISP B and ISP C in this case) doesn't need to see routing (ISP B and ISP C in this case) doesn't need to see routing
information for a single multi-homed customer any differently than information for a single multi-homed customer any differently than a
a singly-homed customer as it has the same routing policy as ISP A singly-homed customer as it has the same routing policy as ISP A
relative to ISP B and ISP C. In other words, with respect to the relative to ISP B and ISP C. In other words, with respect to the
rest of the Internet routing system the organization is singly- rest of the Internet routing system the organization is singly-homed,
homed, so the complexity of the multiple connections is not rele- so the complexity of the multiple connections is not relevant in a
vant in a global sense. Autonomous System Numbers (AS) are iden- global sense. Autonomous System Numbers (AS) are identifiers used in
tifiers used in routing protocols and are needed by routing routing protocols and are needed by routing domains as part of the
domains as part of the global routing system. However, as [4] global routing system. However, as [4] correctly outlines,
correctly outlines, organizations with the same routing policy as organizations with the same routing policy as their upstream provider
their upstream provider do not need an AS. do not need an AS.
Despite this fact, a problem exists in that many ISPs can only Despite this fact, a problem exists in that many ISPs can only
support the load-sharing and reliability requirements of a multi- support the load-sharing and reliability requirements of a multi-
homed customer if that customer exchanges routing information homed customer if that customer exchanges routing information using
using BGP-4 which does require an AS as part of the protocol. BGP-4 which does require an AS as part of the protocol.
2) Singly-homed customers requiring dynamic advertisement of NLRI's 2) Singly-homed customers requiring dynamic advertisement of NLRI's
While this is not a common case as static routing is generally While this is not a common case as static routing is generally
used for this purpose, if a large amount of NLRI's need to be used for this purpose, if a large amount of NLRI's need to be
advertised from the customer to the ISP it is often administra- advertised from the customer to the ISP it is often
tively easier for these prefixes to be advertised using a dynamic administratively easier for these prefixes to be advertised using
routing protocol. Today, the only exterior gateway protocol (EGP) a dynamic routing protocol. Today, the only exterior gateway
that is able to do this is BGP. This leads to the same problem protocol (EGP) that is able to do this is BGP. This leads to the
outlined in condition 1 above. same problem outlined in condition 1 above.
As can be seen there is clearly a problem with the recommendations As can be seen there is clearly a problem with the recommendations
set forth in [4] and the practice of using BGP4 in the scenarios set forth in [4] and the practice of using BGP4 in the scenarios
above. Section 2 proposes a solution to this problem with following above. Section 2 proposes a solution to this problem with following
sections describing the implications and application of the proposed sections describing the implications and application of the proposed
solution. solution.
It should also be noted that if a customer is multi-homed to more It should also be noted that if a customer is multi-homed to more
than one ISP then they are advised to obtain an official allocated AS than one ISP then they are advised to obtain an official allocated AS
from their allocation registry. from their allocation registry.
2. Solution 2. Solution
The solution we are proposing is that all BGP customers homed to the The solution we are proposing is that all BGP customers homed to the
same single ISP use a single, dedicated AS specified by the ISP. same single ISP use a single, dedicated AS specified by the ISP.
Logically, this solution results in an ISP having many peers with the Logically, this solution results in an ISP having many peers with the
same AS, although that AS exists in "islands" completely disconnected same AS, although that AS exists in "islands" completely disconnected
from one another. from one another.
Several practical implications of this solution are discussed in the Several practical implications of this solution are discussed in the
next section. next section.
3. Implications 3. Implications
3.1 Full Routing Table Announcement 3.1 Full Routing Table Announcement
The solution precludes the ability for a BGP customer using the dedi- The solution precludes the ability for a BGP customer using the
cated AS to receive 100% full routes. Because of routing loop detec- dedicated AS to receive 100% full routes. Because of routing loop
tion of AS path, a BGP speaker rejects routes with its own AS number detection of AS path, a BGP speaker rejects routes with its own AS
in the AS path. Imagine Customer X and Customer Y maintain BGP peers number in the AS path. Imagine Customer X and Customer Y maintain
with Provider A using AS number N. Then, Customer X will not be able BGP peers with Provider A using AS number N. Then, Customer X will
to received routes of Customer Y. We do not believe that this would not be able to received routes of Customer Y. We do not believe that
cause a problem for Customer X, though, because Customer X and Cus- this would cause a problem for Customer X, though, because Customer X
tomer Y are both stub networks so default routing is adequate, and and Customer Y are both stub networks so default routing is adequate,
the absence of a very small portion of the full routing table is and the absence of a very small portion of the full routing table is
unlikely to have a noticeable impact on traffic patterns guided by unlikely to have a noticeable impact on traffic patterns guided by
MEDs received. MEDs received.
A BGP customer using the dedicated AS must carry a default route A BGP customer using the dedicated AS must carry a default route
(preferably receiving from its provider via BGP). (preferably receiving from its provider via BGP).
3.2 Change of External Connectivity 3.2 Change of External Connectivity
The dedicated AS specified by a provider is purely for use in peering The dedicated AS specified by a provider is purely for use in peering
between its customers and the provider. When a customer using the between its customers and the provider. When a customer using the
dedicated AS changes its external connectivity, it may be necessary dedicated AS changes its external connectivity, it may be necessary
for the customer to reconfigure their network to use a different AS for the customer to reconfigure their network to use a different AS
number (either a globally unique one if homed to multiple providers, number (either a globally unique one if homed to multiple providers,
or a dedicated AS of a different provider). or a dedicated AS of a different provider).
3.3 Aggregation 3.3 Aggregation
As BGP customers using this dedicated AS are only homed to one ISP, As BGP customers using this dedicated AS are only homed to one ISP,
their routes allocated from its providers CIDR block do not need to their routes allocated from its providers CIDR block do not need to
be announced upstream by its provider as the providers will already be announced upstream by its provider as the providers will already
be originating the larger block. [6]. be originating the larger block. [6].
3.4 Routing Registries 3.4 Routing Registries
The Internet Routing Registry (IRR) [5] is used by providers to gen- The Internet Routing Registry (IRR) [5] is used by providers to
erate route filtering lists. Such lists are derived primarily from generate route filtering lists. Such lists are derived primarily
the "origin" attribute of the route objects. The "origin" is the AS from the "origin" attribute of the route objects. The "origin" is
that originates the route. With multiple customers using the same the AS that originates the route. With multiple customers using the
AS, finer granularity will be necessary to generate the correct route same AS, finer granularity will be necessary to generate the correct
filtering. For example, the "mntner" attribute or the "community" route filtering. For example, the "mntner" attribute or the
attribute of a route object can be used along with the "origin" "community" attribute of a route object can be used along with the
attribute in generating the filtering lists. "origin" attribute in generating the filtering lists.
4. Practice 4. Practice
The AS number specified by a provider can either be an AS from the The AS number specified by a provider can either be an AS from the
private AS space (64512 - 65535) [4], or be an AS previously allo- private AS space (64512 - 65535) [4], or be an AS previously
cated to the provider. With the former, the dedicated AS like all allocated to the provider. With the former, the dedicated AS like
other private AS's should be stripped from its AS path while the all other private AS's should be stripped from its AS path while the
route is being propagated to the rest of the Internet routing system. route is being propagated to the rest of the Internet routing system.
5. Security Considerations 5. Security Considerations
Security considerations are not discussed in this memo. The usage of AS numbers described in this document has no effective
security impact. Acceptance and filtering of AS numbers from
customers is an issue dealt with in other documents.
6. Acknowledgments 6. Acknowledgments
The authors would like to thank Roy Alcala of MCI and Arpakorn The authors would like to thank Roy Alcala of MCI and Arpakorn
Boonkongchuen for their input to this document. The members of the Boonkongchuen for their input to this document. The members of the
IDR Working Group also provided helpful comments. IDR Working Group also provided helpful comments.
7. References 7. References
[1] Rekhter, Y., and Li, T., "A Border Gateway Protocol 4 (BGP-4)", [1] Rekhter, Y., and T. Li, "A Border Gateway Protocol 4 (BGP-4)",
RFC1771, March 1995. RFC 1771, March 1995.
[2] Rekhter, Y., and Gross, P., "Application of the Border Gateway [2] Rekhter, Y., and P. Gross, "Application of the Border Gateway
Protocol in the Internet", RFC1772, March 1995. Protocol in the Internet", RFC 1772, March 1995.
[3] Rekhter, Y., "Routing in a Multi-provider Internet", RFC1787, [3] Rekhter, Y., "Routing in a Multi-provider Internet", RFC 1787,
April 1995. April 1995.
[4] Hawkinson, J., and Bates, T., "Guidelines for creation, selec- [4] Hawkinson, J., and T. Bates, "Guidelines for creation, selection,
tion, and registration of an Autonomous System (AS)", RFC1930, March and registration of an Autonomous System (AS)", RFC 1930, March 1996.
1996.
[5] T. Bates, E. Gerich, L. Joncheray, J-M. Jouanigot, D. Karrenberg, [5] Bates, T., Gerich, E., Joncheray, L., Jouanigot, J-M, Karrenberg,
M. Terpstra, & J. Yu., "Representation of IP Routing Policies in a D., Terpstra, M., and J. Yu., "Representation of IP Routing Policies
Routing Registry (ripe-81++)", RFC1786, March 1995. in a Routing Registry (ripe-81++)", RFC 1786, March 1995.
[6] E. Chen, J. Stewart., "A Framework for Inter-Domain Route Aggre- [6] Chen, E., and J. Stewart., "A Framework for Inter-Domain Route
gation", draft-ietf-idr-aggregation-framework-01.txt, July 1997. Aggregation", Work in Progress.
8. Author's Addresses 8. Authors' Addresses
John Stewart John Stewart
USC/ISI USC/ISI
4350 North Fairfax Drive 4350 North Fairfax Drive
Suite 620 Suite 620
Arlington, VA 22203 Arlington, VA 22203
email: jstewart@isi.edu
Tony Bates EMail: jstewart@isi.edu
Cisco Systems, Inc.
170 West Tasman Drive
San Jose, CA 95134
email: tbates@cisco.com
Ravi Chandra Tony Bates
Cisco Systems, Inc. Cisco Systems, Inc.
170 West Tasman Drive 170 West Tasman Drive
San Jose, CA 95134 San Jose, CA 95134
email: rchandra@cisco.com
Enke Chen EMail: tbates@cisco.com
Cisco Systems, Inc.
170 West Tasman Drive Ravi Chandra
San Jose, CA 95134 Cisco Systems, Inc.
email: enkechen@cisco.com 170 West Tasman Drive
San Jose, CA 95134
EMail: rchandra@cisco.com
Enke Chen
Cisco Systems, Inc.
170 West Tasman Drive
San Jose, CA 95134
EMail: enkechen@cisco.com
9. Full Copyright Statement
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