Network Working Group                                       Peter Psenak
Internet Draft                                            Sina Mirtorabi
Expiration Date: April February 2005                                 Abhay Roy
File name: draft-ietf-ospf-mt-00.txt draft-ietf-ospf-mt-01.txt                         Liem Nguyen
                                                           Cisco Systems
						    Padma Pillay-Esnault
							Juniper Networks

                                                            October 2004

              MT-OSPF: Multi Topology
                                                           Cisco Systems

                                                           February 2005

              Multi-Topology (MT) Routing in OSPF

Status of This Memo

   This document is an Internet-Draft and is in full conformance with subject to all provisions
   of Section 10 section 3 of RFC2026.

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Copyright Notice

   Copyright (C) The Internet Society (2005).

Abstract

   This draft describes the an extension to OSPF in order to define
   independent IP topologies called Multi-Topologies (MTs). The MT
   extension can be used for computing different paths for unicast
   traffic, multicast traffic, different classes of service, or
   in-band management network or incongruent
   topologies for unicast and multicast.  M-ISIS management. [M-ISIS] describes a similar
   mechanism for ISIS.
   This draft also describes an An optional extension of
   Multi-topologies whereby some to exclude
   selected links might be excluded from the default topology. topology is also described.

1. Introduction

   OSPF uses a fixed packet format, therefore it is not easy to
   introduce any backward compatible extensions. However However, the OSPF
   specification [2] [OSPF] introduced TOS metric in an earlier
   specification
   [3] [RFC1583] in order to announce a different link's link cost
   based on TOS. The TOS based routing as described in [3] [RFC1583] was never
   deployed in the field and later was removed from the spec. subsequently deprecated.

   We propose to reuse the TOS based metric fields. They have been
   redefined as MT-ID and MT-ID Metric, Metric and are used to announce advertise
   different topologies by advertising separate metrics for each
   of them.

2. Terminology

   We define the following terminology in this document:

   Non-MT router    : Routers that do not have the MT capability

   MT router        : Routers that have MT capability as described in
                      this document

   MT-ID            : Renamed TOS field in LSAs to represent multi
                      topology ID.

   Default topology : Topology that is built using the TOS 0 metric
                      (default metric)

   MT topology      : Topology that is built using the corresponding
                      MT-ID metric

   MT               : Shorthand notation for MT topology

   MT#0 topology    : Representation of TOS 0 metric in MT-ID format

   Non-MT-Area      : An area that contains only non-MT routers

   MT-Area          : An area that contains both non-MT routers and MT
                      routers or only MT routers

3. MT area boundary

   Each OSPF interface belongs to a single area and all MTs sharing that
   link need to belong to the same area. Therefore the area boundaries
   for all MTs are the same but each MT's attachment to the area is
   independent.

4. Adjacency for MTs

   Each interface can be configured to belong to a set of topologies. A
   single adjacency will be formed with neighbors on the remote neighbor interface
   even if the interface is configured to participate in multiple topologies and
   independently
   topologies.  Furthermore, adjacency formation will be independent
   of the MT-IDs. topologies configured for the interface or neighbors on that
   interface.

5. Sending OSPF control packets

   OSPF control packets should MUST be sent over the default topology.

   OSPF control packets sent to the remote end-point of the a virtual
   link may need to traverse multiple hops. These control packets
   should
   MUST be correctly classified by the virtual link end-point
   routers as packets belonging to the default topology. Event Even though
   the VL may belong to other than
   default topology (or multiple of them), one or more non-default topologies, OSPF control
   packets sent to the remote end of the a virtual link should MUST be forwarded
   using the default topology.

6. Advertising MT adjacencies and corresponding IP prefixes

   We will reuse the TOS metric field in order to announce advertise a topology or
   and prefixes that belongs belonging to a given MT. that topology. The TOS field is renamed to redefined
   as MT-ID in the LSAs payload of Router-LSAs, Summary-LSAs, NSSA-LSAs,
   and AS-External-LSAs (see Appendix A).

   MT-ID metrics in LSAs SHOULD be in ascending order of MT-ID. If
   an MT-ID exists in an LSA or router link multiple times, the metric
   in the first MT-ID instance MUST be used.

6.1 Intra-area routing

   When a router establishes a FULL adjacency over a link that belongs
   to a set of MTs, it will advertise the corresponding cost for each
   MT-ID.

   All

   By default, all links are by default included in default topology, topology and all
   advertised adjacency prefixes belonging to the default topology will use
   the TOS0 metric the same as in standard OSPF. OSPF [OSPF].

   Each MT has its own MT-ID metric field and when field. When a link is not
   part of a given MT, the corresponding MT-ID metric will not appear in is excluded from
   the LSA.

   The Network LSA Network-LSA does not contain any MT information as since the DR is
   shared by all MTs and thus MTs.  Hence, there is no change to the Network LSA. Network-LSA.

6.2 Inter-area and external routing External Routing

   In Summary Summary-LSAs, NSSA-LSAs, and External LSAs, AS-External-LSAs, the TOS metric fields
   field are renamed to defined as MT-ID metric fields and are used in order to announce prefix/router
   advertise prefix and router reachability in the corresponding
   topology.

   When a router originates a type 3/4/5/7 LSA Summary-LSA, NSSA-LSA, or AS-External-LSA
   that belongs to a set of MTs, it will include the corresponding cost
   for each MT-ID. The By default, the router
   by default participate participates in the default
   topology and use uses the TOS0 metric for the default topology the same
   as in standard OSPF. OSPF [OSPF].

   Setting the P-bit in NSSA-LSAs is topology independent and pertains
   to all MT-ID advertised in the body of the LSA.

7. Flushing MT information

   When a certain link/prefix link or prefix that existed or was reachable in a
   certain topology is no longer part of this that topology or the reachability of
   the link/prefix is unreachable
   in this topology was lost, that topology, a new version of the LSA
   that advertised the link/prefix must be originated. This new version
   of the LSA must not include any originated
   excluding metric information representing the
   link/prefix link or prefix in this that
   topology.

   The MT metric in the Router-LSA can also be set to the maximum
   possible metric to enable the router to become a stub in a certain
   topology [4]. [STUB].

8. MT SPF Computation

   By considering MT-ID metrics in the LSAs, OSPF will be able to
   compute multiple topologies, one for each MT the router is part of topologies and find paths to IP prefixes for each MT
   independently. A separate SPF will be computed for each MT-ID to find
   independent paths to IP prefixes. Each nexthop computed during the MT
   SPF MUST belong to the same MT.

   Network LSAs

   Network-LSAs are used by all topologies during the SPF computation.
   During the SPF for a given MT-ID, only the link/metric links and metrics for the given that
   MT-ID will be considered.  Entries in the Router Routing table will
   be MT-ID specific.

   During the SPF computation for the default topology only the TOS0
   metric is considered during the SPF computation.

9. MT ID Values

   Only MT-IDs in the range [0-127] are valid, because external LSAs

   Since AS-External-LSAs use
   one the high order bit in the MT-ID field
   (E bit) for the external metric-type.
   Following metric-type, only MT-IDs in the range
   [0-127] are valid.  The following MT-ID values are reserved:

        0 - reserved Reserved for routers in MTRoutingExclusionCapability mode
            to advertise the metric associated with the default
            topology (see section 11.2).

        1 - reserved Reserved for the default multicast topology.

   Any unknown MT-ID should

   Unknown MT-IDs SHOULD be ignored.

10. Forwarding in MT

   Forwarding must make sure assures that only routes belonging to the a single
   topology are used to forward the a packet along its way from source to
   destination, therefore
   destination. Therefore, user configuration MUST be consistently
   applied throughout the network so that an incoming packet is
   associated with the same topology on through each hop as it is being
   forwarded. end to end.
   It is outside of the scope of this document to consider
   different methods of associating an incoming packet to the a
   corresponding MT. topology.

11. Exclusion of links in the default topology

    The multi-topologies imply that all the routers participate in the
    default topology.  However, it is can be useful in some circumstances to exclude some links
    from the default topology and reserve them for some specific
    classes of traffic.

    The multi-topologies extension for default topology link or prefix
    exclusion is described in the following sections. subsections.

11.1 MT-bit in Hello packet

    OSPF does not have the notion of an unreachable link. All links can
    have a maximum metric of 0xFFFF carried advertised in the Router LSA. Router-LSA. The
    link exclusion capability requires routers to ignore TOS 0 metric TOS0 metrics in
    router-LSA
    Router-LSAs in the default topology and to alternately use the
    MT-ID#0 metric instead to advertise the metric associated with the default
    topology. Hence, all routers within an area MUST agree on how the
    metric for default topology will be advertised.

    The unused T-bit is renamed (MT) defined as the MT-bit in the option field
    in order to
    enforce  assure that a multi-topology link-excluding capable
    router will only interact form an adjacency with another similarly configured
    router.

        +---+---+---+---+---+---+---+---+
        |DN |O  |DC |EA |NP |MC |E  |MT |
        +---+---+---+---+---+---+---+---+

    MT-bit: This bit MUST be set in the Hello packet only if
             MTRoutingExclusionCapability is Enabled enabled (see section 11.2).

11.2 New parameter in the Area Data Structure

   We define a new parameters parameter in the Area Data Structure:

   MTRoutingExclusionCapability
      This is a configurable parameter that will be used to facilitate
      the introduction of MT routers in an area and ensure the backward
      compatibility.

      By default, when an area data structure is created the
      MTRoutingExclusionCapabilty
      MTRoutingExclusionCapability is disabled.

      If MTRoutingExclusionCapability is disabled:
        o The MT-bit MUST be cleared in the Hello packet packets.
        o If a link participates in a non-default topology,
          it is automatically included in the default topology (by using
	  to support backward compatibility between MT and
          non-MT routers. This is accomplished through advertisement
          via the default TOS0 metric field the same as it is done in standard OSPF [2])
	  so that MT routers interact correctly with non-MT routers. [OSPF].

      If MTRoutingExclusionCapability is set to Enabled: enabled:
        o The MT-bit MUST be set in the Hello packet packets
        o The router will only accept a Hello if the MT-bit is set (see
          section 11.3)

      We call MTRoutingExclusionCapability "mode", when

      When MTRoutingExclusionCapability is set to Enabled.

11.3. Forming adjacency enabled a router is
      said to be operating in MTRoutingExclusionCapability mode.

11.3 Adjacency Formation with link exclusion capability. Link Exclusion Capability

   In order to have a smooth transition from a non-MT area to an
   MT-area, a an MT router with MTRoutingExclusionCapability set to disable disabled will
   form
   adjacency adjacencies with non-MT routers and it will include all links
   as part of default topology.

   A link can may cease participating in default topology if
   MTRoutingExclusionCapability is set to Enabled. enabled. In this state, a
   router will only form adjacency with routers that set the MT-bit
   in their Hello packets. This will ensure that all routers are in
   Enabled mode have
   MTRoutingExclusionCapability enabled before the default topology
   can be disabled on a link.

   Receiving OSPF Hello packets as defined in section 8.2 10.5 of [2] are [OSPF] is
   modified as follows:

    o  If the MTRoutingExclusionCapability of the Area Data structure
       is set to Enabled, enabled, the Hello packets are discarded if:

       o if the
       the received Hello packet does not have the MT-bit set

11.4. Sending in the hello
       options set.

11.4 OSPF control packets over an excluded link. Control Packets Transmission Over Excluded Links

   If MTRoutingExclusionCapability is set to Enabled enabled and the default
   topology is not configured on the an interface, connected route should routes MUST
   still exist for a the default topology that would and should enable the OSPF control
   packets to be sent and received.

11.5. Modified MT received over that interface.

11.5 OSPF LSA Advertisement and SPF Computation with link exclusion capability. for Excluded Links

   When MTRoutingExclusionCapability is set to Enabled, MT#0 can be
   removed if a enabled and the link does
   not participate in the default topology. In that
   case topology, the MT-ID#0 metric is not
   advertised. The TOS0 metric is set to infinity (0xFFFF) and but is
   ignored during the MT#0 default topology SPF computation.

   When MTRoutingExclusionCapability is set to Enabled enabled and a link participates
   in the default topology, MT-ID#0 metric is used to advertise the
   metric associated with the default topology. Further The TOS0 metric is set
   to the same value as the MT-ID#0 metric. However TOS 0 metric but is ignored during SPF for the
   default topology and only MT-ID#0 SPF computation.

   Independent of the MTRoutingExclusionCapability setting, the TOS0
   metric is used for SPF in default topology.

   When originating Summary Summary-LSAs, NSSA-LSAs, and External LSAs, if
   MTRoutingExclusionCapability is set to Enabled: AS-External-LSAs.

       o if If the prefix / or router does not exist in the default topology,
         the TOS0 metric is set to infinity (0xFFFFFF).

       o if If the prefix / or router exist exists in default the topology, the
         TOS0 metric is used to announce a prefix / router advertise the metric in the default
         topology.

   During the Summary summary and External external prefix calculation for the default
   topology the TOS0 metric is used in LSA Type-3/4/5/7. for Summary-LSAs, NSSA-LSAs, and
   AS-External-LSAs.

12. Interoperability between MT capable and non-MT capable routers

   The default metric field is mandatory in all LSAs (even when metric
   value is 0). Even when the a link or a prefix does not exist in the
   default topology, a non MT capable non-MT router can consider the zero value
   in the metric field as a valid metric and consider the link/prefix link or
   prefix as part of the default topology.

   In order to prevent the above problem, a an MT capable router will
   by default
   include all links as part of the default topology. If links need
   to be removed from the default topology, a an MT capable router
   MUST be configured in MTRoutingExclusionCapability mode. In this mode
   the router
   mode,  routers will make sure assure that all other routers in the area are
   in the MTRoutingExclusionCapability mode before forming any adjacency so that considering the
   MT-ID#0 metric in the SPF calculation. Only then can the TOS0 metric
   field can be safely ignored during the MT#0 default topology SPF computation.

   Note that for any prefix or router to become reachable in a certain
   topology, a contiguous path inside that topology must exist between
   the calculating router and the destination prefix or router.

13. Migration from non-MT-Area to MT-area

   Introducing MT-OSPF in into a network can be gradually done since gradually to allow
   MT routers will interact and non-MT routers to participate in the default topology with non-MT routers,
   yet exchanging information about other topologies with other
   while MT
   capable routers. routers participate in other topologies.

   If there is a requirement to exclude some links from the default
   topology in an area, all routers in the area MUST be in
   MTRoutingExclusionCapability mode. In this section we describe migrations the
   migration steps to consider while transitioning from a non-MT network
   to a an MT network.

   Migration Steps
   ---------------

   Consider a network with a backbone area and a sets set of non-backbone
   areas functioning in standard OSPF mode. We would like to migrate to
   a
   an MT network either partially or completely.

   1) Part As required, part of an area is upgraded as needed upgrade to have be MT capability, the capable. The
      MT routers will interact with non-MT routers in the default topology,
      further MT routers will
      topology and participate in MT topology other topologies as needed. required.

   2) If a new non-backbone area is created for MT routers, it may be
      set
      configured in MTRoutingExclusionCapability mode as since there is no
      interaction required with non-MT routers, in routers. In this mode mode, the
      default topology can be excluded if on links as required.

   3) If there is more than one non-backbone areas where MT is being
      used, it is desirable that the backbone area 0 be first be upgraded to
      be MT capable
      routers so that inter-area routing is assured for MT
      destinations in different areas.

   4) Gradually the whole network can be made MT aware capable.

   Note that Inter-area inter-area routing for the MT-area still depends on the
   backbone area. Therefore Therefore, if different areas in configured for a given MT-ID
   topology need to communicate, the backbone area also needs to be
   configured for this
   MT-ID. topology.

14. Acknowledgments

   The authors would like to thank Scott Sturgess and Alvaro Retana for
   their comments on the document. Thanks to Acee Lindem for review and
   extensive editing.

15. Security Consideration

   No specific

    The described protocol extension does not introduce any new security
    issues with into the proposed solutions are known. OSPF protocol.

16. IANA Considerations

    The T-bit as defined in [3] [RFC1583] for a router's TOS capability is reclaimed
    redefined as the MT-bit in this document. Likewise, Similarly, the TOS field in type 1,3,4,5,7
    for Router-LSAs, Summary-LSAs, NSSA-LSAs, and AS-External LSAs as
    defined in [2] [OSPF] is reclaimed redefined as MT-ID in this document.

17. Normative References

    [1]

    [M-ISIS]  Przygienda, T., Shen, N., Sheth, N.,
              "Multi Topology (MT) Routing in IS-IS",
              draft-ietf-isis-wg-multi-topology-06.txt,
              Work in progress.

    [2]

    [OSPF]    Moy, J., "OSPF Version 2", RFC 2328, April 1998.

    [3]

    [RFC1583] Moy, J., "OSPF Version 2", RFC 1583, Proteon, Inc., March 1994.

    [4]

    [NSSA]    Murphy, P., "The OSPF Not-So-Stubby Area (NSSA) Option",
              RFC 3101, January 2003.

    [RFC2119] Bradner, S., "Key words for use in RFC's to Indicate
              Requirement Levels", RFC 2328, March 1977.

18. Informative References

   [STUB]    Retana, A., Nguyen, L., White, R., Zinin, A. and D.
             McPherson, "OSPF Stub Router Advertisement", RFC 3137, June
             2001.

Appendix A.

   LSAs

   LSA content defined in [2] are [OSPF] is modified to introduce the MT-ID.

A.1 Router-LSAs

   Router-LSAs are the Type 1 LSAs.  Each router in an area originates
   a router-LSA.  The LSA describes the state and cost of the router's
   links (i.e., interfaces) to the area.  All of the router's links to
   the area must be described in a single router-LSA.  For details
   concerning the construction of router-LSAs, see Section 12.4.1. [2]
   12.4.1 [OSPF].

      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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |            LS age             |     Options   |       1       |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                        Link State ID                          |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                     Advertising Router                        |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                     LS sequence number                        |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |         LS checksum           |             length            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |*|*|*|N|W|V|E|B|        0      |            # links            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                          Link ID                              |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                         Link Data                             |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |     Type      |     # MT-ID   |            metric             |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |     MT-ID     |       0       |          MT-ID  metric        |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                              ...                              |

     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |     MT-ID     |       0       |          MT-ID  metric        |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                          Link ID                              |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                         Link Data                             |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                              ...                              |

A.2 Network-LSAs

   Network-LSAs are the Type 2 LSAs.  A network-LSA is originated for
   each broadcast and NBMA network in the area which supports two or
   more routers.  The network-LSA is originated by the network's
   Designated Router.  The LSA describes all routers attached to the
   network, including the Designated Router itself.  The LSA's Link
   State ID field lists the IP interface address of the Designated
   Router.

   The distance from the network to all attached routers is zero. This
   is why metric fields need not be specified in the network-LSA. For
   details concerning the construction of network-LSAs, see Section
   12.4.2. [2]
   12.4.2 [OSPF].

      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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |            LS age             |      Options  |      2        |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                        Link State ID                          |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                     Advertising Router                        |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                     LS sequence number                        |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |         LS checksum           |             length            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                         Network Mask                          |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                        Attached Router                        |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                              ...                              |

   Note that network LSA does not contain any MT-ID field fields as the cost
   of the network to the attached routers is 0 and DR is shared by
   all MT. topologies.

A.3 Summary-LSAs

   Summary-LSAs are the Type 3 and 4 LSAs.  These LSAs are originated
   by area border routers. Summary-LSAs describe inter-area
   destinations.  For details concerning the construction of summary-
   LSAs, see Section 12.4.3. [2] 12.4.3 [OSPF].

   Type 3 summary-LSAs are used when the destination is an IP network.
   In this case the LSA's Link State ID field is an IP network number
   (if necessary, the Link State ID can also have one or more of the
   network's "host" bits set; see Appendix E [2] [OSPF] for details). When
   the destination is an AS boundary router, a Type 4 summary-LSA is
   used, and the Link State ID field is the AS boundary router's OSPF
   Router ID.  (To see why it is necessary to advertise the location of
   each ASBR, consult Section 16.4 of [2]). [OSPF]). Other than the difference
   in the Link State ID field, the format of Type 3 and 4 summary-LSAs
   is identical.

       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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |            LS age             |     Options   |    3 or 4     |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                        Link State ID                          |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                     Advertising Router                        |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                     LS sequence number                        |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |         LS checksum           |             length            |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                         Network Mask                          |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |       0       |                  metric                       |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |     MT-ID     |                MT-ID  metric                  |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                              ...                              |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |     MT-ID     |                MT-ID  metric                  |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

A.4.5 AS-external-LSAs

   AS-external-LSAs are the Type 5 LSAs. These LSAs are originated by
   AS boundary routers, and describe destinations external to the AS.
   For details concerning the  construction of AS-external-LSAs, see
   Section 12.4.3. [2] 12.4.3 [OSPF].

   AS-external-LSAs usually describe a particular external destination.
   For these LSAs the Link State ID field specifies an IP network number
   (if necessary, the Link State ID can also have one or more of the
   network's "host" bits set; see Appendix E [2] [OSPF] for details).  AS-
   external-LSAs
   AS-external-LSAs are also used to describe a default route.  Default
   routes are used when no specific route exists to the destination.
   When describing a default route, the Link State ID is always set to
   DefaultDestination (0.0.0.0) and the Network Mask is set to 0.0.0.0.

       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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |            LS age             |     Options   |      5        |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                        Link State ID                          |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                     Advertising Router                        |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                     LS sequence number                        |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |         LS checksum           |             length            |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                         Network Mask                          |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |E|     0       |                  metric                       |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                      Forwarding address                       |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                      External Route Tag                       |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |E|    MT-ID    |              MT-ID  metric                    |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                      Forwarding address                       |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                      External Route Tag                       |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                              ...                              |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |E|    MT-ID    |              MT-ID  metric                    |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                      Forwarding address                       |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                      External Route Tag                       |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

A.4.6 NSSA-LSAs

   NSSA-LSAs are the Type 7 LSAs. These LSAs are originated by
   AS boundary routers local to an NSSA, and describe destinations
   external to the AS. The changes to NSSA-LSAs are identical to those
   for External-LSAs (Appendix A.4.5).  For details concerning the
   construction of NSSA-LSAs see Section 2.4 [NSSA].

Authors' address

   Peter Psenak                         Abhay Roy
   Cisco Systems                        Cisco systems
   Parc Pegasus,                        170 W. Tasman Dr.
   De Kleetlaan 6A                      San Jose, CA 95134
   1831 Diegem, Belgium                 USA
   E-mail: ppsenak@cisco.com            E-mail: akr@cisco.com

   Sina Mirtorabi                       Liem Nguyen
   Cisco Systems                        Cisco Systems
   225 West Tasman drive                7025 Kit Creek Rd.
   San Jose, CA 95134                   Research Triangle Park, NC 27709
   USA                                  USA
   E-mail: sina@cisco.com               E-mail: lhnguyen@cisco.com

   Padma PIllay-Esnault
   Juniper Networks
   1194 N. Mathilda Avenue
   Sunnyvale, Pillay-Esnault
   Cisco Systems
   3750 Cisco Way
   San Jose, CA 94089 95134
   USA
   E-mail: padma@juniper.net ppe@cisco.com

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