Network Working Group Alvaro Retana Internet Draft Liem Nguyen Expiration Date: March 2001 Russ White File name: draft-ietf-ospf-stub-adv-00.txt Alex Zinin Cisco Systems Danny McPherson Amber Networks September 2000 OSPF Stub Router Advertisement draft-ietf-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 in order to discourage transit traffic from entering the router. The mechanism proposed here requires no OSPF protocol changes and is completely interoperable with the existing OSPF specification. Retana, Nguyen, White, McPherson, Zinin [Page 1]
INTERNET DRAFT OSPF Stub Router Advertisement July 2000 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 (e.g., 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 discourages other routers from using it for transit routing, while still forwarding packets to router's own IP addresses (i.e., the router is announced as a non-forwarding, or overloaded router, to borrow a term from the IS-IS protocol). 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 non-forwarding (or overloaded) 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 when 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. Retana, Nguyen, White, McPherson, Zinin [Page 2]
INTERNET DRAFT OSPF Stub Router Advertisement July 2000 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 backup link), and because router X will not have links to its neighbors. To address all three problems, router X announces its router-LSA to the neighbors as follows. o Costs of all non-stub links (links which are not type 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 non-forwarding router. If the path through the non-forwarding router is the only available path, the routers of the first type will not use it for transit (which is the desired behavior), while the routers Retana, Nguyen, White, McPherson, Zinin [Page 3]
INTERNET DRAFT OSPF Stub Router Advertisement July 2000 of the second type will still use this path. Deployment Considerations Such a mechanism increases overall network availablity and allows network operators to alleviate the deterministic blackholing behavoir introduced in this scenario. The IS-IS Overload Bit [ISO10589] has been employed in IS-IS routing domains to achieve similar behavior. Triggers for setting the link costs as described are left to the implementor. Some potential triggers could include N seconds after booting, or N number of BGP prefixes in the BGP Loc-RIB. Also, understand that this mechanism assumes actual deployments assign substantially lower values for link costs (and the sum of sub- sequent path costs), and that a value of 0xFFFF for an individual link within a path would be sufficiently large enough to discourage transit traffic from entering the router. 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 discus- sions around this topic and Alex Zinin for his work on this draft as well. 6 Security Considerations The technique described in this document does not introduce any new security issues into OSPF protocol. 7 References [RFC1247] J. Moy. OSPF version 2. RFC 1247, July 1991 [RFC1583] J. Moy. OSPF version 2. RFC 1583, March 1994 [RFC2328] J. Moy. OSPF version 2. Technical Report RFC 2328, April 1998 [ISO10589] "Intermediate System to Intermediate System Intra- Retana, Nguyen, White, McPherson, Zinin [Page 4]
INTERNET DRAFT OSPF Stub Router Advertisement July 2000 Domain Routeing Exchange Protocol for use in Conjunction with the Protocol for Providing the Connectionless-mode Network Service (ISO 8473)", ISO DP 10589, February 1990. 8 Authors' Addresses Alvaro Retana Liem Nguyen 7025 Kit Creek Rd. 7025 Kit Creek Rd. Research Triangle Park, NC 27709 Research Triangle Park, NC 27709 USA USA e-mail: aretana@cisco.com e-mail: lhnguyen@cisco.com Russ White Danny McPherson Cisco Systems, Inc. Amber Networks, Inc. 7025 Kit Creek Rd. 2465 Augustine Drive Research Triangle Park, NC 27709 Santa Clara, CA 95054 e-mail: riw@cisco.com e-mail: danny@ambernetworks.com Alex Zinin Cisco Systems, Inc. 170 W. Tasman Dr. San Jose, CA 95134 e-mail: azinin@cisco.com Retana, Nguyen, White, McPherson, Zinin [Page 5]
