How to avoid error messages for non-RFC8950 peers

[André Grüneberg]

Hi,

We have lately enabled RFC8950 (BGP IPv4 routes with IPv6 nexthop) on (one
of) our route servers. We did this for all the connected peers.

This means two things: We added ipv4 channels to "protocol bgp"s with an
IPv6 neighbor and we enabled "extended next hop" in these channels. This
will announce IPv4 and the respective extended next hop capability in BGP
OPEN.

In an ideal world all existing peers would only have IPv6 AFI enabled on
their IPv6 BGP sessions. Unfortunately some BGP implementations by default
have proxy-arp^WIPv4 AFI enabled for all neighbors. So we ended up with
some peers announcing IPv4 AFI but not "extended next hop".

Obviously Bird now tries to send those peers all the IPv4 routes it has in
the routing table. Since the route server is transparent, the routes have
an IPv6 next-hop and thus we get the following error messages:
2025-08-21 08:14:24 <ERR> pb_0359_asXXXX: Invalid NEXT_HOP attribute -
mismatched address family (2001:7f8:19:1:0:3:48d2:1 for ipv4)
2025-08-21 08:14:24 <ERR> pb_0359_asXXXX: Invalid route 45.91.12.0/24
withdrawn

Of course I'll try to educate people to correct their configuration to get
closer to the ideal world above. At the same time, I'd love to avoid those
error messages.

So I've been thinking how to avoid the situation we are in and came up with
two potential approaches:

   1. Since all session are configured in passive mode, the neighbors
   should send their BGP OPEN first. So upon receiving IPv4 AFI without
   receiving "Extended next hop - IPv6 nexthop: ipv4" we will remove IPv4 AFI
   from our BGP OPEN answer.
   2. If we had the negotiated protocol capabilities available in the
   export filter, we could filter IPv4 routes with IPv6 nexthop in case
   extended next hop is not negotiated.

Currently I only see "require extended next hop" switch, but to my
understanding this will tear down the BGP session -- which is undesirable.

Does anyone have other good ideas?

BTW: It's not very intuitive to understand from the above error messages
that the announcement is egress. :)

Best wishes,
André

-- 
André Grüneberg, Managing Director
andre.grueneberg at bcix.de
+49 30 2332195 42

BCIX Management GmbH
Albrechtstr. 110
12103 Berlin
Germany

Geschäftsführer/Managing Directors: Jens Lietzmann, André Grüneberg
Handelsregister: Amtsgericht Charlottenburg, HRB 143581 B
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