[Docs] [txt|pdf] [Tracker] [WG] [Email] [Diff1] [Diff2] [Nits]

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

Network Working Group                                       Peter Psenak
Internet Draft                                            Sina Mirtorabi
Expiration Date: September 2005                                Abhay Roy
File name: draft-ietf-ospf-mt-02.txt                         Liem Nguyen
                                                    Padma Pillay-Esnault
                                                           Cisco Systems

                                                              March 2005


              Multi-Topology (MT) Routing in OSPF


Status of This Memo

   This document is an Internet-Draft and is subject to all provisions
   of section 3 of RFC 3667.  By submitting this Internet-Draft, each
   author represents that any applicable patent or other IPR claims of
   which he or she is aware have been or will be disclosed, and any of
   which he or she become aware will be disclosed, in accordance with
   RFC 3668.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups.  Note that
   other groups may also distribute working documents as
   Internet-Drafts.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   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.

   This Internet-Draft will expire on August 22, 2005.

Copyright Notice

   Copyright (C) The Internet Society (2005).

Abstract

   This draft describes 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 network management. [M-ISIS] describes a similar
   mechanism for ISIS. An optional extension to exclude
   selected links from the default topology is also described.


Psenak et al.                                                   [Page 1]

Internet Draft                MT-OSPF                     September 2005



1. Introduction

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

   We propose to reuse the TOS based metric fields. They have been
   redefined as MT-ID and MT-ID Metric and are used to 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.


Psenak et al.                                                   [Page 2]

Internet Draft                MT-OSPF                     September 2005


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 interface
   even if the interface is configured to participate in multiple
   topologies.  Furthermore, adjacency formation will be independent
   of the topologies configured for the interface or neighbors on that
   interface.

5. Sending OSPF control packets

   OSPF control packets MUST be sent over the default topology.

   OSPF control packets sent to the remote end-point of a virtual
   link may need to traverse multiple hops. These control packets
   MUST be correctly classified by the virtual link end-point
   routers as packets belonging to the default topology. Even though
   the VL may belong to one or more non-default topologies, OSPF control
   packets sent to the remote end of a virtual link 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 advertise a topology
   and prefixes belonging to that topology. The TOS field is redefined
   as MT-ID in the 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.

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

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

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


Psenak et al.                                                   [Page 3]

Internet Draft                MT-OSPF                     September 2005



6.2 Inter-area and External Routing

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

   When a router originates a 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. By default, the router participates in the default
   topology and uses the TOS0 metric for the default topology the same
   as in standard 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 or prefix that existed or was reachable in a
   certain topology is no longer part of that topology or is unreachable
   in that topology, a new version of the LSA must be originated
   excluding metric information representing the link or prefix in 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 [STUB].


8. MT SPF Computation

   By considering MT-ID metrics in the LSAs, OSPF will be able to
   compute multiple 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 are used by all topologies during the SPF computation.
   During the SPF for a given MT-ID, only the links and metrics for 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.


Psenak et al.                                                   [Page 4]

Internet Draft                MT-OSPF                     September 2005



9. MT ID Values

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

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

        1 - Reserved for the default multicast topology.

   Unknown MT-IDs SHOULD be ignored.


10. Forwarding in MT

   Forwarding assures that only routes belonging to a single
   topology are used to forward a 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 through each hop end to end.
   It is outside of the scope of this document to consider
   different methods of associating an incoming packet to a
   corresponding topology.

Psenak et al.                                                   [Page 5]

Internet Draft                MT-OSPF                     September 2005



11. Exclusion of links in the default topology

    The multi-topologies imply that all the routers participate in the
    default topology.  However, it can be useful 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 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 advertised in the Router-LSA. The
    link exclusion capability requires routers to ignore TOS0 metrics in
    Router-LSAs in the default topology and to alternately use the
    MT-ID#0 metric 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 defined as the MT-bit in the option field
    in order to  assure that a multi-topology link-excluding capable
    router will only 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 (see section 11.2).


11.2 New parameter in the Area Data Structure

   We define a new 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 backward
      compatibility.




Psenak et al.                                                   [Page 6]

Internet Draft                MT-OSPF                     September 2005



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

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

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

      When MTRoutingExclusionCapability is set to enabled a router is
      said to be operating in MTRoutingExclusionCapability mode.


11.3 Adjacency Formation with Link Exclusion Capability

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

   A link may 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 have
   MTRoutingExclusionCapability enabled before the default topology
   can be disabled on a link.

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

    o  If the MTRoutingExclusionCapability of the Area Data structure
       is set to enabled, the Hello packets are discarded if the
       the received Hello packet does not have the MT-bit in the hello
       options set.

11.4 OSPF Control Packets Transmission Over Excluded Links

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



Psenak et al.                                                   [Page 7]

Internet Draft                MT-OSPF                     September 2005





11.5 OSPF LSA Advertisement and SPF Computation for Excluded Links

   When MTRoutingExclusionCapability is enabled and the link does
   not participate in the default topology, the MT-ID#0 metric is not
   advertised. The TOS0 metric is set to infinity (0xFFFF) but is
   ignored during the default topology SPF computation.

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


   Independent of the MTRoutingExclusionCapability setting, the TOS0
   metric is used for Summary-LSAs, NSSA-LSAs, and AS-External-LSAs.

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

       o If the prefix or router exists in default the topology, the
         TOS0 metric is used to advertise the metric in the default
         topology.

   During the summary and external prefix calculation for the default
   topology the TOS0 metric is used 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 a link or prefix does not exist in the
   default topology, a non-MT router can consider the zero value
   in the metric field as a valid metric and consider the link or
   prefix as part of the default topology.

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



Psenak et al.                                                   [Page 8]

Internet Draft                MT-OSPF                     September 2005



   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 into a network can be done gradually to allow
   MT routers and non-MT routers to participate in the default topology
   while MT 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 the
   migration steps to consider while transitioning from a non-MT network
   to an MT network.

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

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

   2) If a new non-backbone area is created for MT routers, it may be
      configured in MTRoutingExclusionCapability mode since there is no
      interaction required with non-MT routers. In this mode, the
      default topology can be excluded 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 first be upgraded to
      be MT capable so that inter-area routing is assured for MT
      destinations in different areas.

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

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


Psenak et al.                                                   [Page 9]

Internet Draft                MT-OSPF                     September 2005


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

    The described protocol extension does not introduce any new security
    issues into the OSPF protocol.


16. IANA Considerations

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




17. Normative References

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

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

    [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 2119, March 1997.



18. Informative References

   [M-ISIS]  Przygienda, T., Shen, N., Sheth, N.,
             "M-ISIS: Multi Topology (MT) Routing in IS-IS",
             draft-ietf-isis-wg-multi-topology-07.txt,
             Work in progress.

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


Psenak et al.                                                  [Page 10]

Internet Draft                MT-OSPF                     September 2005


Appendix A.

   LSA content defined in [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 [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                             |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                              ...                              |





Psenak et al.                                                  [Page 11]

Internet Draft                MT-OSPF                     September 2005



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












Psenak et al.                                                  [Page 12]

Internet Draft                MT-OSPF                     September 2005



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 [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 [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 [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                  |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+








Psenak et al.                                                  [Page 13]

Internet Draft                MT-OSPF                     September 2005


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 [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 [OSPF] 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                       |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


Psenak et al.                                                  [Page 14]

Internet Draft                MT-OSPF                     September 2005



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
   Cisco Systems
   3750 Cisco Way
   San Jose, CA 95134
   USA
   E-mail: ppe@cisco.com


Full Copyright Statement

   Copyright (C) The Internet Society (2004).  This document is subject
   to the rights, licenses and restrictions contained in BCP 78, and
   except as set forth therein, the authors retain all their rights.

   This document and the information contained herein are provided on an
   "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
   OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
   ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
   INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
   INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
   WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.





Psenak et al.                                                  [Page 15]

Internet Draft                MT-OSPF                     September 2005




Intellectual Property

   The IETF takes no position regarding the validity or scope of any
   Intellectual Property Rights or other rights that might be claimed to
   pertain to the implementation or use of the technology described in
   this document or the extent to which any license under such rights
   might or might not be available; nor does it represent that it has
   made any independent effort to identify any such rights.  Information
   on the procedures with respect to rights in RFC documents can be
   found in BCP 78 and BCP 79.

   Copies of IPR disclosures made to the IETF Secretariat and any
   assurances of licenses to be made available, or the result of an
   attempt made to obtain a general license or permission for the use of
   such proprietary rights by implementers or users of this
   specification can be obtained from the IETF on-line IPR repository at
   http://www.ietf.org/ipr.

   The IETF invites any interested party to bring to its attention any
   copyrights, patents or patent applications, or other proprietary
   rights that may cover technology that may be required to implement
   this standard.  Please address the information to the IETF at ietf-
   ipr@ietf.org.

Acknowledgement

   Funding for the RFC Editor function is currently provided by the
   Internet Society.

Psenak et al.                                                  [Page 16]


Html markup produced by rfcmarkup 1.107, available from http://tools.ietf.org/tools/rfcmarkup/