RFC 5004

Avoid BGP Best Path Transitions from One External to Another

📅 published September 2007, from ietf

Avoid BGP Best Path Transitions from One External to Another 🦋

okay bestie, so this one is an Standards Track doc from September 2007 (RFC 5004 💅), written by Enke Chen and Srihari Sangli at Cisco Systems ✨. It's about BGP route selection, and it's literally so good. It's giving "stop the churn" energy 🌸

Status of This Memo

This doc specifies an Internet standards track protocol for the Internet community, and it's asking for discussion and suggestions for improvements 🫶. Check 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

okay so here's the vibe 💖. We're proposing an extension to the BGP route selection rules that avoids unnecessary best path transitions between external paths under certain conditions. It helps overall network stability, and the real iconic part is that it eliminates certain BGP route oscillations where more than one external path from one BGP speaker contributes to the churn 😭🔥.

1. Introduction ✨

The last two steps of BGP route selection (Section 9.1.2.2, [BGP]) compare the BGP identifiers and the peering addresses 🧚. The BGP identifier (treated either as an IP address or just an integer [BGP-ID]) is allocated by the Autonomous System (AS) that the speaker belongs to. So for a local BGP speaker, the BGP identifier of a route received from an external peer is just a random number 🎲. When the routes under consideration come from external peers, the result of the last two steps is "random" as far as the local BGP speaker is concerned 🙈.

This observation is the whole reason we propose an extension to the BGP route selection rules that avoids unnecessary best-path transitions between external paths under certain conditions 🍓. It helps the overall network stability, and more importantly, it eliminates 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 💜

Picture this: the existing best path A is from an external peer, and then another external path B gets selected as the new best path by the route selection algorithm 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. That way we avoid 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) 👯. The peering addresses for the parallel sessions are typically allocated by one AS (possibly with route selection considerations), so the algorithm (if applied) could impact the existing routing setup 😬. Not applying it also keeps the allocation of peering addresses as a simple and effective tool for influencing route selection when parallel BGP sessions exist 💅🌈.

4. The Benefits 🌟

The extension avoids unnecessary best-path transitions between external paths under certain conditions, and it clearly helps reduce routing and forwarding changes in a network 📉. That means better overall network stability 🫶.

But the main event is the oscillation fix 🔥. Check out the example in Figure 1, where:

R1, R2, R3, and R4 belong to one AS.
R1 is a route reflector with R3 as its client.
R2 is a route reflector with R4 as its client.
The IGP metrics are as listed.
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

Note however, there are permanent BGP route oscillation scenarios [RFC3345] that this mechanism does NOT eliminate 🙅‍♀️. Not every drama gets resolved 😭.

So here's the tea ☕. Because of how route reflection [BGP-RR] interacts with 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's enough to stop the route oscillation 🛑💖.

Other route reflection enhancements are being developed that could also eliminate this type of oscillation. Still, the proposed algorithm is extremely simple, can be implemented and deployed immediately, and doesn't introduce any backward compatibility issues 🏆✨.

5. Remarks 💅

The proposed algorithm is backward-compatible, and it can be deployed on a per-BGP-speaker basis 🎀. Deployment is highly recommended on a BGP speaker with multiple external BGP peers (especially the ones connecting to an inter-exchange point) 🌸.

Real talk, compared to the existing behavior, the proposed algorithm may introduce some "non-determinism" in BGP route selection 🤷‍♀️. Although, one can argue that the BGP Identifier comparison in the existing route selection already introduced some "randomness," as described in the introduction 🙈. That "non-determinism" hasn't been shown to be detrimental in practice, and it can be completely eliminated by using the existing mechanisms (such as setting LOCAL_PREF or MED) if you want 💜.

6. Security Considerations 🔐

This extension does not introduce any security issues 🛡️. Slay, no drama here 💀✨.

7. Acknowledgments 🫶

The idea 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

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