BIRD is not prepending ASN on EBGP export.

[Maria Matejka]

Hello,

you basically want one router to prepend two ASNs for one hop,
if I understand correctly. That's not an intuitive / default-worthy
behavior.

On Thu, Oct 10, 2024 at 10:20:51AM +0000, Ochalski, Radoslaw via Bird-users wrote:

> Here you have same route exported via Juniper border, local ASN 20940 is prepended by default:
> 
> inet.0: 955317 destinations, 2138917 routes (950415 active, 218793 holddown, 14419 hidden)
>   Prefix                  Nexthop              MED     Lclpref    AS path
> * 20.135.0.0/16           Self                                    20940 8075 I
> 
> vs bird:
> 
>         BGP.as_path: 8075
> 
> [truncated]

BIRD shows the route after filters but _before_ BGP applies the export
modifications, which includes prepending the local AS to the AS path iff
the session is EBGP. I expect that Juniper is showing the route
as-it-goes-to-the-wire.

BIRD 2 doesn't allow showing routes that way; this is possible in BIRD 3
with `export table on` by showing routes from the export table.

If you capture tcpdump from BIRD, you'll see the AS prepended.

Maria

-- 
Maria Matejka (she/her) | BIRD Team Leader | CZ.NIC, z.s.p.o.
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