On Tue, Jul 07, 2026 at 07:03:38PM +0800, xinzhe liu via Bird-users wrote: > Hello BIRD maintainers, > > I would like to ask for clarification about BIRD's BGP `merge paths` > equivalence rules. > > BIRD 2.19.0+ appears to use an AS_PATH-length-based equivalence check for > BGP > routes when kernel `merge paths` is enabled. In the reproduced cases, BIRD > exported one ECMP route from two BGP paths even though: > > 1. The two eBGP paths came from different neighboring ASes, and their > AS_PATH > contents were different. > 2. The two iBGP paths had different AS_PATH contents. > > The common condition was that both paths had the same AS_PATH length and > otherwise equal attributes. This looks similar to AS-path multipath-relax > behavior in implementations such as FRR, but in FRR this relaxed behavior is > enabled by an explicit `bgp bestpath as-path multipath-relax` option. FRR's > default multipath behavior is stricter: eBGP multipath requires the same > remote AS, and iBGP multipath requires matching AS_PATH contents.
Hello Yes, this is expected behavior. The 'merge paths' option merges paths where one is not clearly better than other, regardless of specifics of their AS paths. It would make sense to have better configuration granularity for this, but it is low on our todo list. -- Elen sila lumenn' omentielvo Ondrej 'Santiago' Zajicek (email: [email protected]) "To err is human -- to blame it on a computer is even more so."
