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Versions: 00 01 02 03 04 05 06 07 08 09 RFC 4915

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

                                                    Padma Pillay-Esnault
                                                        Juniper Networks

                                                            October 2004



              MT-OSPF: Multi Topology (MT) Routing in OSPF



Status of This Memo

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

   Internet Drafts are working documents of the Internet Engineering
   Task Force (IETF), its Areas, and its Working Groups. Note that other
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   The list of current Internet-Drafts can be accessed at
   http://www.ietf.org/ietf/1id-abstracts.txt

   The list of Internet-Draft Shadow Directories can be accessed at
   http://www.ietf.org/shadow.html.

Abstract

   This draft describes the 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 different
   classes of service, in-band management network or incongruent
   topologies for unicast and multicast.  M-ISIS describes a similar
   mechanism for ISIS.
   This draft also describes an optional extension of
   Multi-topologies whereby some links might be excluded from the
   default topology.



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1. Introduction

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

   We propose to reuse the TOS based metric fields. They have been
   redefined as MT-ID and MT-ID Metric, to announce 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#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 the remote neighbor even if the
   interface is configured to participate in multiple topologies and
   independently of the MT-IDs.

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5. Sending OSPF control packets

   OSPF control packets should be sent over the default topology.

   OSPF control packets sent to the remote end-point of the virtual
   link may need to traverse multiple hops. These control packets
   should be correctly  classified by the routers as packets belonging
   to the default topology. Event though the VL may belong to other than
   default topology (or multiple of them), OSPF control packets sent to
   the remote end of the virtual link should 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 a topology or
   prefixes that belongs to a given MT. The TOS field is renamed to
   MT-ID in the LSAs payload (see Appendix A).


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 links are by default included in default topology, all
   advertised adjacency belonging to the default topology will use
   the TOS0 metric as in standard OSPF.

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

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


6.2 Inter-area and external routing

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

   When a router originates a type 3/4/5/7 LSA that belongs to a set of MTs,
   it will include the corresponding cost for each MT-ID. The router
   by default participate in default topology and use the TOS0 metric
   for default topology as in standard OSPF.




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7. Flushing MT information

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


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

   During the SPF computation for default topology 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 use
   one bit in the MT-ID field (E bit) for the external metric-type.
   Following MT-ID values are reserved:

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

        1 - reserved for default multicast topology.

   Any unknown MT-ID should be ignored.


10. Forwarding in MT

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

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11. Exclusion of links in the default topology

    The multi-topologies imply that all the routers participate in the
    default topology. However, it is 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 exclusion
    is described in the following sections.


11.1 MT-bit in Hello packet

    OSPF does not have the notion of unreachable link. All links can
    have a maximum metric of 0xFFFF carried in the Router LSA. The link
    exclusion capability requires routers to ignore TOS 0 metric in
    router-LSA in default topology and use 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) in the option field in order to
    enforce that a multi-topology link-excluding capable router will
    only interact 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 (see section 11.2).


11.2 New parameter in the Area Data Structure

   We define a new parameters 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 is disabled.

      If MTRoutingExclusionCapability is disabled:
        o MT-bit MUST be cleared in the Hello packet
        o If a link participates in a non-default topology,
          it is automatically included in default topology (by using
          the default metric field as it is done in standard OSPF [2])
          so that MT routers interact correctly with non-MT routers.



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

      We call MTRoutingExclusionCapability "mode", when
      MTRoutingExclusionCapability is set to Enabled.

11.3. Forming adjacency with link exclusion capability.

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

   A link can cease participating in default topology if
   MTRoutingExclusionCapability is set to 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 before default topology can be disabled on a link.

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

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

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


11.4. Sending OSPF control packets over an excluded link.

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


11.5. Modified MT SPF Computation with link exclusion capability.

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

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


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   When originating Summary and External LSAs, if
   MTRoutingExclusionCapability is set to Enabled:

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

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

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


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 link or a prefix does not exist in the
   default topology, a non MT capable router can consider the zero value
   in the metric field as a valid metric and consider the link/prefix as
   part of the default topology.

   In order to prevent the above problem, a 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 MT capable router
   MUST be configured in MTRoutingExclusionCapability mode. In this mode
   the router will make sure that all routers in the area are in the
   MTRoutingExclusionCapability mode before forming any adjacency so that
   TOS0 metric field can be safely ignored during the MT#0 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 a network can be gradually done since MT
   routers will interact in default topology with non-MT routers,
   yet exchanging information about other topologies with other MT
   capable routers.

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

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

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   1) Part of an area is upgraded as needed to have MT capability, the
      MT routers will interact with non-MT routers in default topology,
      further MT routers will participate in MT topology as needed.

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

   3) If there is more than one non-backbone areas where MT is being
      used, it is desirable that area 0 be first upgraded to 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

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


14. Acknowledgments

   The authors would like to thank Scott Sturgess and Alvaro Retana for
   their comments on the document.


15. Security Consideration

   No specific security issues with the proposed solutions are known.


16. IANA Considerations

   T-bit defined in [3] for router's TOS capability is reclaimed as
   MT-bit in this document. Likewise, TOS field in type 1,3,4,5,7 LSAs
   defined in [2] is reclaimed as MT-ID in this document.


17. References

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

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

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

    [4] Retana, Nguyen, White, "OSPF Stub Router Advertisement",
        RFC 3137, June 2001.







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Appendix A.

   LSAs content defined in [2] are modified to introduce 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]


      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                             |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                              ...                              |





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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]

      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 as the cost of
   the network to the attached routers is 0 and DR is shared by all MT.












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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]


   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] 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]). 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                  |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+








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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]

   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] for details).  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                       |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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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, CA 94089
   USA
   E-mail: padma@juniper.net





























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