Network Working Group Enke Chen Internet Draft Redback Networks Expiration Date: December 2004 Srihari R. Sangli Procket Networks Avoid BGP Best Path Transitions from One External to Another draft-ietf-idr-avoid-transition-00.txt 1. 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 and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as ``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. 2. Abstract In this document we propose a revision to the BGP route selection rules that would avoid unnecessary best path transitions between external paths under certain conditions. The proposed revision would help the overall network stability, and more importantly, would eliminate certain BGP route oscillations in which more than one external paths from one router contribute to the churn. Chen & Sangli [Page 1] Internet Draft draft-ietf-idr-avoid-transition-00.txt May 2004 3. Introduction The last two steps of the BGP route selection (Sect. 9.1.2.2, [1]) involve comparing the BGP identifiers and the peering addresses. The BGP identifier, (treated either as an IP address, or just an integer [2]) for a BGP speaker is allocated by the AS to which the speaker belongs. As a result, for a local BGP speaker, the BGP identifier of a route received from an external peer is just an random number. When routes under consideration are from external peers, the result from the last two steps of the route selection is therefore "random" as far as the local BGP speaker is concerned. It is based on this observation that we propose a revision to the BGP route selection rules that would avoid unnecessary best path transitions between external paths under certain conditions. The proposed revision would help the overall network stability, and more importantly, would eliminate certain BGP route oscillations in which more than one external paths from one router contribute to the churn. 4. Specification of Requirements The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119 [5]. 5. The Algorithm Consider the case in which the existing best path A is from an external peer, and another external path B is then selected as the new best path by the route selection algorithm described in [1]. When neither Path A nor Path B is eliminated by the route selection algorithm prior to Step f) - BGP identifier comparison (Sect. 9.1.2.2 [1]), we propose that the existing best path (Path A) be kept as the best path (thus avoiding switching the best path to Path B). This algorithm SHOULD NOT be applied when either path is from a BGP Confederation peer. In addition, the algorithm SHOULD NOT be applied when both paths are from peers with identical BGP identifier (i.e., there exist parallel BGP sessions between two routers). As the peering addresses for the parallel sessions are typically allocated by one AS (possibly with route selection considerations), the algorithm (if applied) could impact the existing routing setup. Furthermore, by not applying the algorithm, the allocation of peering addresses would remain as a simple and effective tool in influencing route selection when Chen & Sangli [Page 2] Internet Draft draft-ietf-idr-avoid-transition-00.txt May 2004 parallel BGP sessions exist. 6. The Benefits The proposed revision to the BGP route selection rules avoids unnecessary best path transitions between external paths under certain conditions. Clearly the revision would help reduce routing and forwarding changes in a network, thus help the overall network stabilities. More importantly, as shown in the following example, the proposed revision can be used to eliminate certain BGP route oscillations in which more than one external paths from one router contribute to the churn. Note however, that there are permanent BGP route oscillation scenarios [3] that the mechanism described in this document does not eliminate. Consider the example in Fig. 1 where o R1, R2, R3 and R4 belong to one AS o R1 is a route reflector with R3 as its client. o R2 is a route reflector with R4 as its client. o The IGP metrics are as listed. o External paths (a), (b) and (c) are as described in Fig. 2. +----+ 40 +----+ | R1 |--------------| R2 | +----+ +----+ | | | | | 10 | 10 | | | | +----+ +----+ | R3 | | R4 | +----+ +----+ / \ | / \ | (a) (b) (c) Figure 1 Path AS MED Identifier a 1 0 2 b 2 20 1 Chen & Sangli [Page 3] Internet Draft draft-ietf-idr-avoid-transition-00.txt May 2004 c 2 10 5 Figure 2 Due to the interaction of route reflection [4] and MEDs, the best path on R1 keeps churning between (a) and (c), and the best path on R3 keeps churning between (a) and (b). With the proposed algorithm R3 would not switch the best path from (a) to (b) even after R1 withdraws (c) toward its clients, and that is enough to stop the route oscillation. Although this type of route oscillations can also be eliminated by other route reflection enhancements being developed, the proposed algorithm is very simple and can be deployed immediately. 7. Remarks The proposed algorithm is backward-compatible, and can be deployed on a per-node basis. The deployment of the algorithm is highly recommended on a router with multiple external BGP peers (especially the ones connecting to an inter-exchange point). Compared to the existing behavior, the proposed algorithm may introduce some "non-determinism" in the BGP route selection - although one can argue that the BGP Identifier comparison in the existing route selection has already introduced some "randomness" as described in the introduction section. Such "non-determinism" or "randomness" has not been shown to be detrimental in practice, and can be completely eliminated by using the existing mechanisms (such as setting LOCAL_PREF or MED) if so desired. 8. Security Considerations This extension does not introduce any security issues. Chen & Sangli [Page 4] Internet Draft draft-ietf-idr-avoid-transition-00.txt May 2004 9. Acknowledgments The idea presented was inspired by a route oscillation case observed on the BBN/Genuity backbone in 1998 while both authors were at Cisco Systems. The algorithm was also implemented at that time. The authors would like to thank Yakov Rekhter and Ravi Chandra for their comments on the initial idea. 10. References [1] Y. Rekhter, T. Li, and S. Hares, "A Border Gateway Protocol 4 (BGP-4)", draft-ietf-idr-bgp4-23.txt, November 2003. [2] E. Chen and J. Yuan, "AS-wide Unique BGP Identifier for BGP-4", , December 2003. [3] D. McPherson, V, Gill, D. Walton, and A. Retana, "Border Gateway Protocol (BGP) Persistent Route Oscillation Condition", RFC 3345, August 2002. [4] T. Bates, R. Chandra, and E. Chen, "BGP Route Reflection - An Alternative to Full Mesh IBGP", RFC 2796, April 2000. [5] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. 11. Author Information Enke Chen Redback Networks, Inc. 300 Holger Way San Jose, CA 95134 e-mail: enke@redback.com Srihari R. Sangli Procket Networks, Inc. 1100 Cadillac Court Milpitas, CA 95035 e-mail: srihari@procket.com Chen & Sangli [Page 5]