EBGP Multihop /w Directly Connected Peer

[Anthony Hoppe]

Hello,

I have a situation where I need to enable EBGP multihop even though the
peer is directly connected.  Backstory: I have hosts running BIRD peered
with MLAG paired switches and there are instances where BGP ECMP and LACP
hashing don't align.  BGP packets destined to one peer (Switch A) will land
on the other switch in the pair (Switch B).  In this case, the switch in
which the packet landed (Switch B) will route the packet to the correct
switch (Switch A) causing TTL to decrement.

When I configure multihop in BIRD, it seems BIRD assumes that the next-hop
address is unreachable and expects there to be a static route to the peer
before it will consider accepted routes reachable.  I can't add a static
route that points to itself, so in my testing environment I ended up adding
a /128 to the loopback interface on both switches and configured BIRD peer
with that address.  Then I was able to add the static route BIRD is
expecting (and is now actually necessary), and multihop works fine.

However, if possible, I'd like to avoid having to do this.  Is there a way?

Thanks!

~ Anthony

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