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PROPOSED STANDARD
Network Working Group                                            E. Chen
Request for Comments: 5004                                     S. Sangli
Category: Standards Track                                  Cisco Systems
                                                          September 2007
Avoid BGP Best Path Transitions from One External to Another

Status of This Memo

This document specifies an Internet standards track protocol for the Internet community, and requests discussion and suggestions for improvements. Please refer to the current edition of the "Internet Official Protocol Standards" (STD 1) for the standardization state and status of this protocol. Distribution of this memo is unlimited.

Abstract

In this document, we propose an extension to the BGP route selection rules that would avoid unnecessary best path transitions between external paths under certain conditions. The proposed extension would help the overall network stability, and more importantly, would eliminate certain BGP route oscillations in which more than one external path from one BGP speaker contributes to the churn.
1.  Introduction
The last two steps of the BGP route selection (Section 9.1.2.2, [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 Autonomous System (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 a 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 an extension to the BGP route selection rules that would avoid unnecessary best-path transitions between external paths under certain conditions. The proposed extension would help the overall network stability, and more importantly, would eliminate certain BGP route oscillations in which more than one external path from one BGP speaker contributes to the churn.
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RFC 5004                Best BGP Route Selection          September 2007
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 (Section 9.1.2.2, [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 an 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 extension to the BGP route selection rules avoids unnecessary best-path transitions between external paths under certain conditions. Clearly, the extension would help reduce routing and forwarding changes in a network, thus helping the overall network stability.

More importantly, as shown in the following example, the proposed extension can be used to eliminate certain BGP route oscillations in which more than one external path from one BGP speaker contributes 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 Figure 1 where
Chen & Sangli               Standards Track                     [Page 2]

RFC 5004                Best BGP Route Selection          September 2007
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 Figure 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 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 oscillation 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.
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RFC 5004                Best BGP Route Selection          September 2007
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.  Security Considerations
This extension does not introduce any security issues.
7.  Acknowledgments
The idea presented was inspired by a route oscillation case observed in the BBN/Genuity network 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.
8.  Normative References
[BGP] Rekhter, Y., Ed., Li, T., Ed., and S. Hares, Ed., "A Border
Gateway Protocol 4 (BGP-4)", RFC 4271, January 2006.

[BGP-RR] Bates, T., Chen, E., and R. Chandra, "BGP Route Reflection:
An Alternative to Full Mesh Internal BGP (IBGP)", RFC 4456, April 2006.

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
9.  Informative References
[BGP-ID] Chen, E. and J. Yuan, "AS-wide Unique BGP Identifier for
BGP-4", Work in Progress, November 2006.
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RFC 5004                Best BGP Route Selection          September 2007
[RFC3345] McPherson, D., Gill, V., Walton, D., and A. Retana, "Border
Gateway Protocol (BGP) Persistent Route Oscillation Condition", RFC 3345, August 2002.

Author Information

Enke Chen Cisco Systems, Inc. 170 W. Tasman Dr. San Jose, CA 95134

EMail: enkechen@cisco.com


Srihari R. Sangli Cisco Systems, Inc. 170 W. Tasman Dr. San Jose, CA 95134

EMail: rsrihari@cisco.com
Chen & Sangli               Standards Track                     [Page 5]

RFC 5004                Best BGP Route Selection          September 2007
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Chen & Sangli Standards Track [Page 6]

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