We have a weird routing loop that I cannot explain. I believe it's a software bug, but just want to be sure.
I have 2 routers A and B (Extreme X480) connected together in the same AS and exchanging full routing table without any filter.
Router A got one transit link announcing full Internet routing table. Router A is sending his local prefix only (one /22) to that transit provider.
Router B got another transit link announcing full Internet routing table. Router B is sending his local prefix only (one /22) to that transit provider.
Simple multihomed topology.
I am having issue with 1 IP of one /20 Internet prefix.
This /20 is prefered on Router B, because AS-PATH is shorter.
From Router B, I can traceroute, ping and everything to the whole /20, no matter the source.
From Router A, I can traceroute, ping for the whole /20 via Router B, EXCEPT for one IP of that /20. That IP is looping between Router A and Router B.
I searched for policy, filter, static route or something that could explain why Router B is sending back the traffic to Router A only for this specific destination .. but nothing. The BGP table and routing table yet say to route that destination to the transit provider on Router B.
In other word, have you ever seen a router send traffic to a destination that is not indicated in the routing / BGP table ?
Thanks for anwsers / ideas