Introduction

1. Avoid BGP Best Path Transition from One External to Another Enke Chenenke@redback.comSrihari R. Sanglisrihari@procket.com

2. Introduction • BGP Identifier is allocated by the operator of an AS • Subject to change • Appears as a “random” number to another AS • The last two steps of route selection involves comparing the BGP Identifiers • “random” when the comparison is between two external paths • Network stability can be improved in certain cases by not transitioning the best path

3. The Algorithm • We can avoid changing the best path when: • There exist an old best path and a new best path based on the complete route selection algorithm. • Both paths are valid and are from external peers. • Neither path is eliminated by the route selection steps prior to the BGP identifier comparison. • Exceptions • when either path is from a confederation peer • Do not apply the algorithm for parallel sessions.

4. Benefits • It helps select a more stable external path. • It helps eliminate ibgp route oscillations when the best path keeps switching between two external paths. • Useful when there are multiple external peers (common for the Interconnection Points) • Simple and effective fix • Deployed to resolve a route oscillation case in a larger provider network

5. Misc • E. Chen and S. Sangli, “Avoid BGP Best Path Transition from One External to Another”, draft-chen-bgp-avoid-transition-00.txt • Request the draft be adopted as an IDR Working Group document.