Internet DRAFT - draft-ietf-ospf-stub-adv


Network Working Group                                      Alvaro Retana
Internet Draft                                               Liem Nguyen
Expiration Date: May 2001                                     Russ White
File name: draft-ietf-ospf-stub-adv-02.txt                    Alex Zinin
                                                           Cisco Systems
                                                         Danny McPherson
                                                          Amber Networks
                                                           November 2000

                     OSPF Stub Router Advertisement

Status of this Memo

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

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   In some cases, it is desirable not to route transit traffic via a
   specific OSPF router.  However, OSPF [RFC2328] does not specify a
   standard way to accomplish this. This memo describes a backward-
   compatible technique that may be used by OSPF implementations to
   advertise unavailability to forward transit traffic or to lower the
   preference level for the paths through such a router.

1 Motivation

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   In some situations, it may be advantageous to inform routers in a
   network not to use a specific router as a transit point, but still
   route to it. Possible situations include the following.

       o    The router is in a critical condition (for example, has very
            high CPU load or does not have enough memory to store all
            LSAs or build the routing table).

       o    Graceful introduction and removal of the router to/from the

       o    Other (administrative or traffic engineering) reasons.

   Note that the proposed solution does not remove the router from the
   topology view of the network (as could be done by just flushing that
   router's router-LSA), but prevents other routers from using it for
   transit routing, while still routing packets to router's own IP
   addresses, i.e., the router is announced as stub.

   It must be emphasized that the proposed solution provides real bene-
   fits in networks designed with at least some level of redundancy so
   that traffic can be routed around the stub router. Otherwise, traffic
   destined for the networks reachable through such a stub router will
   be still routed through it.

2 Proposed Solution

   The solution described in this document solves two challenges associ-
   ated with the outlined problem. In the description below, router X is
   the router announcing itself as a stub.

      1)   Making other routers prefer routes around router X while per-
           forming the Dijkstra calculation.

      2)   Allowing other routers to reach IP prefixes directly con-
           nected to router X

   Note that it would be easy to address issue 1) alone by just flushing
   router X's router-LSA from the domain. However, it does not solve
   problem 2), since other routers will not be able to use links to
   router X in Dijkstra (no back link), and because router X will not
   have links to its neighbors.

   To address both problems, router X announces its router-LSA to the
   neighbors as follows.

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      o    costs of all non-stub links (links of the types other than 3)
           are set to LSInfinity (16-bit value 0xFFFF, rather than 24-
           bit value 0xFFFFFF used in summary and AS-external LSAs).

      o    costs of stub links (type 3) are set to the interface output

   This addresses issues 1) and 2).

3 Compatibility issues

   Some inconsistency may be seen when the network is constructed of the
   routers that perform intra-area Dijkstra calculation as specified in
   [RFC1247] (discarding link records in router-LSAs that have LSInfin-
   ity cost value) and routers that perform it as specified in [RFC1583]
   and higher (do not treat links with LSInfinity cost as unreachable).
   Note that this inconsistency will not lead to routing loops, because
   if there are some alternate paths in the network, both types of
   routers will agree on using them rather than the path through the
   stub router. If the path through the stub router is the only one, the
   routers of the first type will not use the stub router for transit
   (which is the desired behavior), while the routers of the second type
   will still use this path.

4 Acknowledgements

   The authors of this document do not make any claims on the original-
   ity of the ideas described.  Among other people, we would like to
   acknowledge Henk Smit for being part of one of the initial discus-
   sions around this topic.

5 Security Considerations

   The technique described in this document does not introduce any new
   security issues into OSPF protocol.

6 References

      J. Moy. OSPF version 2. Technical Report RFC 2328,
       Internet Engineering Task Force, 1998.


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      J. Moy. OSPF version 2. RFC 1247,
       Internet Engineering Task Force, 1991.

      J. Moy. OSPF version 2. RFC 1583,
       Internet Engineering Task Force, 1994.

8 Authors' addresses

   Alvaro Retana
   7025 Kit Creek Rd.
   Research Triangle Park, NC 27709

   Liem Nguyen
   7025 Kit Creek Rd.
   Research Triangle Park, NC 27709

   Russ White
   Cisco Systems, Inc.
   7025 Kit Creek Rd.
   Research Triangle Park, NC 27709

   Alex D. Zinin
   Cisco Systems
   150 West Tasman Dr.
   San Jose,CA 95134

   Danny McPherson
   Amber Networks
   2465 Augustine Drive
   Santa Clara, CA  95054

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