A funny story with limited benefit that someone else may learn from someday...

[Jake]

For 20 years in the industry, I’ve misunderstood an element of BGP.

My understanding was that “multihop” was used any time you might receive a 
route from a thing, but be given destinations for other things.

In some cases, that mean routers multiple hops away, yes.  But in my 
incorrect understanding, it also meant route-servers at Internet 
Exchanges (which are considered directly connected.

So for all of my time at this job and last, I set nearly everything in 
Quagga/zebra/BIRD to multihop…and it worked.

But…recently something in Linux changed, it seems…where the algorithm used 
to recursively try to solve routes when “multihop” was used now completely 
ignores directly-connected interfaces, maybe...?

So…when BIRD did it’s reconfig…all of a sudden it was unable to resolve 
the route lookups for anything labeled “multihop”.  This resulted in every 
route received over a multihop BGP session with a route-server at an IXP 
being marked "!", unreachable, reject.

Thus, removing the line “multihop” from all of our route-server peers 
fixed the issue, routes were again received and added to the kernel 
properly, and life went on.

Maybe nobody else out there has the same misunderstanding I did, and 
nobody will ever experience this again...but if posting here in my 
embarrassment helps one of you down the line, I suppose it was all worth 
it.

-jake