Network Working Group                                          A. Retana
Request for Comments: 3137                                     L. Nguyen
Category: Informational                                         R. White
                                                          Cisco Systems
                                                               A. Zinin
                                                          Nexsi Systems
                                                           D. McPherson
                                                         Amber Networks
                                                              June 2001


                    OSPF Stub Router Advertisement

Status of this Memo

  This memo provides information for the Internet community.  It does
  not specify an Internet standard of any kind.  Distribution of this
  memo is unlimited.

Copyright Notice

  Copyright (C) The Internet Society (2001).  All Rights Reserved.

Abstract

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

1. Motivation

  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
        network.

     o  Other (administrative or traffic engineering) reasons.





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  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
  benefits 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
  associated 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
        performing the Dijkstra calculation.

     2) Allowing other routers to reach IP prefixes directly connected
        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.

     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
        cost.

  This addresses issues 1) and 2).










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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
  LSInfinity 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
  originality of the ideas described.  Among other people, we would
  like to acknowledge Henk Smit for being part of one of the initial
  discussions around this topic.

5. Security Considerations

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

6. References

  [RFC2328] Moy, J., "OSPF Version 2", STD 54, RFC 2328, April 1998.

  [RFC1247] Moy, J., "OSPF Version 2", RFC 1247, July 1991.

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

















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7. Authors' Addresses

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

  EMail: [email protected]


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

  EMail: [email protected]


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

  EMail: [email protected]


  Alex Zinin
  Nexsi Systems
  1959 Concourse Drive
  San Jose,CA 95131

  EMail: [email protected]


  Danny McPherson
  Amber Networks
  48664 Milmont Drive
  Fremont, CA 94538

  EMail: [email protected]











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8. Full Copyright Statement

  Copyright (C) The Internet Society (2001).  All Rights Reserved.

  This document and translations of it may be copied and furnished to
  others, and derivative works that comment on or otherwise explain it
  or assist in its implementation may be prepared, copied, published
  and distributed, in whole or in part, without restriction of any
  kind, provided that the above copyright notice and this paragraph are
  included on all such copies and derivative works.  However, this
  document itself may not be modified in any way, such as by removing
  the copyright notice or references to the Internet Society or other
  Internet organizations, except as needed for the purpose of
  developing Internet standards in which case the procedures for
  copyrights defined in the Internet Standards process must be
  followed, or as required to translate it into languages other than
  English.

  The limited permissions granted above are perpetual and will not be
  revoked by the Internet Society or its successors or assigns.

  This document and the information contained herein is provided on an
  "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
  TASK FORCE DISCLAIMS 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.

Acknowledgement

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



















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