Carlos Mendioroz sent me a seemingly simple question: when is a BGP route invalid? My knee-jerk reaction: when the next hop is not reachable (and I’m not the only one). WRONG – BGP routes with unreachable next hop are still valid, as shown in the following printout:
R1#show ip bgp 10.1.1.0
BGP routing table entry for 10.1.1.0/24, version 6
Paths: (1 available, no best path)
Not advertised to any peer
192.168.0.1 (inaccessible) from 172.16.1.2 (192.168.0.1)
Origin IGP, metric 0, localpref 100, valid, internal
Has anyone seen a BGP route that was not valid recently? Could you tell us how you got it? I’m suspecting you might get an invalid route with RPKI, but don’t have the necessary infrastructure in place to test it.