I wanted to write a blog post explaining the intricacies of Advertisement of Multiple Paths in BGP, got into a yak-shaving exercise when discussing the need to exchange BGP capabilities to enable this feature, and decided to turn it into a separate prerequisite blog post. The optimal path selection with BGP AddPath post is coming in a few days.

The Problem

Whenever you want to use BGP for something else than simple IPv4 unicast routing the BGP neighbors must agree on what they are willing to do – be it multiprotocol extensions and individual additional address families, graceful restart, route refresh… (IANA has the complete BGP Capability Codes registry).

The Dynamic Negotiation of BGP Capabilities blog post generated almost no comments, apart from the #facepalm realization that a certain network operating system resets IBGP sessions when the sole EBGP session goes down, but there were a few interesting comments on LinkedIn and Twitter.

While most engineers easily relate to the awkwardness of bringing down a BGP session to enable new functionality (Tearing down BGP session, as a solution reminds me rebooting a host, as a solution.), it’s not as easy as it looks. As Adam Chappell put it “Dynamic capability renegotiation does tend to sound a bit like changing the tyres while still moving. Very neat if you can pull it off but so much to go wrong…”