Network Working Group Alex Zinin Internet Draft Alvaro Retana Expiration Date: January 2001 Liem Nguyen File name: draft-zinin-ospf-stub-adv-00.txt Russ White Cisco Systems July 2000 OSPF Stub Router Advertisement draft-zinin-ospf-stub-adv-00.txt Status of this Memo This document is an Internet-Draft and is in full conformance with all provisions of Section 10 of RFC2026. 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. Internet Drafts may be updated, replaced, or obsoleted by other documents at any time. It is not appropriate to use Internet Drafts as reference material or to cite them other than as a "working draft" or "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. Abstract 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 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. Zinin [Page 1]
INTERNET DRAFT OSPF Stub Router Advertisement July 2000 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. 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 either be still routed through it or black-holed (see Section 4 for more details). 2 Proposed Solution The solution described in this document solves three challenges asso- ciated 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 3) Allowing router X itself to use its links to reach remote routers and destinations. 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 problems 2) and 3), 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 all three problems, router X announces its router-LSA to Zinin [Page 2]
INTERNET DRAFT OSPF Stub Router Advertisement July 2000 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). To address issue 3), router X must use real link costs in its Dijkstra calculation. This will allow it to use its links to reach remote routers. 3 Implementation details To simplify the implementation of the described technique, it may be useful for OSPF routers to keep two versions of their router-LSAs. One version (public) is installed in the LSDB and sent to the neigh- bors, while another version (internal) is used for local routing table calculation. When the router announces itself as stub, it con- structs the public version as indicated in Section 2, but the inter- nal version is constructed as in standard OSPF [RFC2328]. When the router does not announce itself as stub, both versions are con- structed as in standard OSPF and would both yield the same result, hence it is not necessary (though acceptable) to keep two versions of the LSAs at this point. 4 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. 5 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 Zinin [Page 3]
INTERNET DRAFT OSPF Stub Router Advertisement July 2000 discussions around this topic. 6 Security Considerations The technique described in this document does not introduce any new security issues into OSPF protocol. 7 References [RFC2328] J. Moy. OSPF version 2. Technical Report RFC 2328, Internet Engineering Task Force, 1998. ftp://ftp.isi.edu/in-notes/rfc2328.txt. [RFC1247] J. Moy. OSPF version 2. RFC 1247, Internet Engineering Task Force, 1991. ftp://ftp.isi.edu/in-notes/rfc1247.txt. [RFC1583] J. Moy. OSPF version 2. RFC 1583, Internet Engineering Task Force, 1994. ftp://ftp.isi.edu/in-notes/rfc1583.txt. 8 Authors' addresses Alex D. Zinin Liem Nguyen Cisco Systems 7025 Kit Creek Rd. 150 West Tasman Dr. Research Triangle Park, NC 27709 San Jose,CA 95134 USA E-mail: azinin@cisco.com e-mail: lhnguyen@cisco.com Alvaro Retana Russ White 7025 Kit Creek Rd. Cisco Systems, Inc. Research Triangle Park, NC 27709 7025 Kit Creek Rd. USA Research Triangle Park, NC 27709 e-mail: aretana@cisco.com e-mail: riw@cisco.com Zinin [Page 4]