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.

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

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.

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.
   
   [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

Full Copyright Statement

Copyright © The IETF Trust (2007).

This document is subject to the rights, licenses and restrictions contained in BCP 78, and except as set forth therein, the authors retain all their rights.

This document and the information contained herein are provided on an "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Intellectual Property

The IETF takes no position regarding the validity or scope of any Intellectual Property Rights or other rights that might be claimed to pertain to the implementation or use of the technology described in this document or the extent to which any license under such rights might or might not be available; nor does it represent that it has made any independent effort to identify any such rights. Information on the procedures with respect to rights in RFC documents can be found in BCP 78 and BCP 79.

Copies of IPR disclosures made to the IETF Secretariat and any assurances of licenses to be made available, or the result of an attempt made to obtain a general license or permission for the use of such proprietary rights by implementers or users of this specification can be obtained from the IETF on-line IPR repository at http://www.ietf.org/ipr.

The IETF invites any interested party to bring to its attention any copyrights, patents or patent applications, or other proprietary rights that may cover technology that may be required to implement this standard. Please address the information to the IETF at ietf-ipr@ietf.org.