GENWiki

Premier IT Outsourcing and Support Services within the UK

User Tools

Site Tools


rfc:rfc3137

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.

Retana, et al. Informational [Page 1] RFC 3137 OSPF Stub Router Advertisement June 2001

 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).

Retana, et al. Informational [Page 2] RFC 3137 OSPF Stub Router Advertisement June 2001

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.

Retana, et al. Informational [Page 3] RFC 3137 OSPF Stub Router Advertisement June 2001

7. Authors' Addresses

 Alvaro Retana
 7025 Kit Creek Rd.
 Research Triangle Park, NC 27709
 USA
 EMail: aretana@cisco.com
 Liem Nguyen
 7025 Kit Creek Rd.
 Research Triangle Park, NC 27709
 USA
 EMail: lhnguyen@cisco.com
 Russ White
 Cisco Systems, Inc.
 7025 Kit Creek Rd.
 Research Triangle Park, NC 27709
 EMail: riw@cisco.com
 Alex Zinin
 Nexsi Systems
 1959 Concourse Drive
 San Jose,CA 95131
 EMail: azinin@nexsi.com
 Danny McPherson
 Amber Networks
 48664 Milmont Drive
 Fremont, CA 94538
 EMail: danny@ambernetworks.com

Retana, et al. Informational [Page 4] RFC 3137 OSPF Stub Router Advertisement June 2001

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.

Retana, et al. Informational [Page 5]

/data/webs/external/dokuwiki/data/pages/rfc/rfc3137.txt · Last modified: 2001/07/03 20:01 by 127.0.0.1

Donate Powered by PHP Valid HTML5 Valid CSS Driven by DokuWiki