Network Working Group E. Chen Internet Draft S. Sangli Expiration Date:
June 2006September 2007 Cisco Systems Avoid BGP Best Path Transitions from One External to Another draft-ietf-idr-avoid-transition-04.txtdraft-ietf-idr-avoid-transition-05.txt Status of this Memo By submitting this Internet-Draft, each author represents that any applicable patent or other IPR claims of which he or she is aware have been or will be disclosed, and any of which he or she becomes aware will be disclosed, in accordance with Section 6 of BCP 79. 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. Abstract In this document we propose a revisionan extension to the BGP route selection rules that would avoid unnecessary best path transitions between external paths under certain conditions. The proposed revisionextension would help the overall network stability, and more importantly, would eliminate certain BGP route oscillations in which more than one external pathspath from one BGP speaker contributecontributes to the churn. 1. Introduction The last two steps of the BGP route selection (Sect. 126.96.36.199, [BGP]) involve comparing the BGP identifiers and the peering addresses. The BGP identifier (treated either as an IP address, or just an integer [BGP-ID]) 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 revisionan extension to the BGP route selection rules that would avoid unnecessary best path transitions between external paths under certain conditions. The proposed revisionextension would help the overall network stability, and more importantly, would eliminate certain BGP route oscillations in which more than one external pathspath from one BGP speaker contributecontributes to the churn. 2. 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 [RFC2119]. 3. 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 [BGP]. When comparing all the paths in route selection, if neither Path A nor Path B is eliminated by the route selection algorithm prior to Step f) - BGP identifier comparison (Sect. 188.8.131.52 [BGP]), 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 BGP speakers). 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 parallel BGP sessions exist. 4. The Benefits The proposed revisionextension to the BGP route selection rules avoids unnecessary best path transitions between external paths under certain conditions. Clearly the revisionextension would help reduce routing and forwarding changes in a network, thus help the overall network stabilities.stability. More importantly, as shown in the following example, the proposed revisionextension can be used to eliminate certain BGP route oscillations in which more than one external pathspath from one BGP speaker contributecontributes to the churn. Note however, that there are permanent BGP route oscillation scenarios [RFC3345] 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 c 2 10 5 Figure 2 Due to the interaction of the route reflection [BGP-RR] and MEDs,the MULTI_EXIT_DISC (MED) attribute, 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 extremely simple and can be implemented and deployed immediately without introducing any backward compatibility issues. 5. Remarks The proposed algorithm is backward-compatible, and can be deployed on a per-BGP-speaker basis. The deployment of the algorithm is highly recommended on a BGP speaker 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" 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. 6. IANA Considerations This extension does not require any action by IANA. 7. Security Considerations This extension does not introduce any security issues. 8. Acknowledgments The idea presented was inspired by a route oscillation case observed on the BBN/Genuity backbone in 1998. The algorithm was also implemented and deployed at that time. The authors would like to thank Yakov Rekhter and Ravi Chandra for their comments on the initial idea. 9. Normative References [BGP] Y.Rekhter, T.Y., Li, T., and S.Hares, S., "A Border Gateway Protocol 4 (BGP-4)", draft-ietf-idr-bgp4-26.txt, October 2004.RFC 4271, January 2006. [BGP-RR] T. Bates, R. Chandra, and E. Chen, "BGP Route Reflection - An Alternative to Full Mesh IBGP", RFC 2796,4456, April 2000.2006. [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. 10. Non-normative References [BGP-ID] E. Chen and J. Yuan, "AS-wide Unique BGP Identifier for BGP-4", Work in Progress, draft-ietf-idr-bgp-identifier-06.txt,draft-ietf-idr-bgp-identifier-08.txt, November 2005.2006. [RFC3345] D. McPherson, V, Gill, D. Walton, and A. Retana, "Border Gateway Protocol (BGP) Persistent Route Oscillation Condition", RFC 3345, August 2002. 11. Author Information Enke Chen Cisco Systems, Inc. 170 W. Tasman Dr. San Jose, CA 95134 Email: firstname.lastname@example.org Srihari R. Sangli Cisco Systems, Inc. 170 W. Tasman Dr. San Jose, CA 95134 Email: email@example.com 12. 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