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Network Working Group                                          A. RetanaRequest for Comments: 3137                                     L. NguyenCategory: Informational                                         R. White                                                           Cisco Systems                                                                A. Zinin                                                           Nexsi Systems                                                            D. McPherson                                                          Amber Networks                                                               June 2001OSPF Stub Router AdvertisementStatus 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 20013. 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 20017. 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.comRetana, et al.               Informational                      [Page 4]

RFC 3137             OSPF Stub Router Advertisement            June 20018. 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]