Cisco BGP identifiers (posted 2016-04-22)

One of the tiebreakers in the BGP best path selection algorithm is to prefer the path learned from a BGP speaker with the lowest BGP identifier. So how are BGP identifier selected when they're not configured explicitly?

I always forget whether it's the highest or the lowest IP address configured on a Cisco router. Turns out this is remarkably hard to find in Google, but if you know where to look it's in Cisco's IOS command reference:

• If a loopback interface is configured, the router ID is set to the IP address of the loopback interface. If multiple loopback interfaces are configured, the router ID is set to the IP address of the loopback interface with the highest IP address.

• If no loopback interface is configured, the router ID is set to the highest IP address on a physical interface.