FW: BIRD is not prepending ASN on EBGP export.

[Alexander Zubkov]

Hi Radoslaw,

Do I get it right, that you have 2 bgp peerings here. First your route
passes this peering:
AS8075 <-> AS20940
then it passes other peering:
AS4290006033 <-> AS4290006002

Then it is an expected behaviour. Because the ASN is prepended when
route is exported over eBGP session. And the local ASN is prepended.
So you see that when you receive the route over the first peering, it
is prepended with AS8075. Then it goes over the second peering, where
it is prepended with AS4290006033.

Regards,
Alexander

On Tue, Oct 8, 2024 at 4:33 PM Ochalski, Radoslaw via Bird-users
<bird-users at network.cz> wrote:
>
> Hi,
>
>
>
> We have observed strange behavior where BIRD is not adding ASN on EBGP export.
>
> Route is received on IBGP session within AS20940:
>
>
>
> r04.border01.lon03.fab > bird show  config template bgp 4INTERECOR_ROUTERS
>
> template bgp '4INTERECOR_ROUTERS' {
>
>     local as 20940;
>
>     neighbor as 20940;
>
>
>
> r04.border01.lon03.fab> show route protocol 23.210.49.60__r24.lon02.ien for 20.135.0.0/16 all
>
> Executing "/usr/sbin/birdc show route protocol '23.210.49.60__r24.lon02.ien' for 20.135.0.0/16 all"
>
> Table master4:
>
> 20.135.0.0/16        unicast [23.210.49.60__r24.lon02.ien 2024-10-07] (100) [AS8075i]
>
>         via 23.210.49.60 on ae1
>
>         Type: BGP univ
>
>         BGP.origin: IGP
>
>         BGP.as_path: 8075
>
>         BGP.next_hop: 23.210.49.60
>
>         BGP.med: 4294967294
>
>         BGP.local_pref: 350
>
>
>
> Route ASPATH: 8075 which is expected.
>
>
>
>
>
> We are exporting this route on EBGP session from private ASN 4290006033 <-> to private ASN: 4290006002:
>
>
>
> r04.border01.lon03.fab> show route export 2a02:26f0:1880:3702::1__r01.stem01 for 20.135.0.0/16 all
>
> Executing "/usr/sbin/birdc show route export '2a02:26f0:1880:3702::1__r01.stem01' for 20.135.0.0/16 all"
>
> Table master4:
>
> 20.135.0.0/16        unicast [2.22.144.250__r01.sdn 2024-10-07 from 2.22.144.250] * (100) [AS8075i]
>
>         via 23.210.49.60 on ae1
>
>         Type: BGP univ
>
>         BGP.origin: IGP
>
>         BGP.as_path: 8075
>
>         BGP.next_hop: 23.210.49.60
>
>         BGP.med: 4294967294
>
>         BGP.local_pref: 777
>
>
>
> Route is received on 4290006002:
>
> r01.stem01.lon03.fab> show route protocol 2a02:26f0:1880:3702::__r04.border01 for 20.135.0.0/16  all
>
> Executing "/usr/sbin/birdc show route protocol '2a02:26f0:1880:3702::__r04.border01' for 20.135.0.0/16 all"
>
> Table master4:
>
> 20.135.0.0/16        unicast [2a02:26f0:1880:3702::__r04.border01 2024-10-07] * (100) [AS8075i]
>
>         via 2a02:26f0:1880:3702:: on ae4
>
>         Type: BGP univ
>
>         BGP.origin: IGP
>
>         BGP.as_path: 4290006033 8075
>
>         BGP.next_hop: 2a02:26f0:1880:3702:: fe80::b6db:91ff:fe8b:1eb4
>
>         BGP.local_pref: 100
>
>
>
> ASPATH: 4290006033 8075
>
>
>
> We are missing AS 20940 on which essentially this prefix is received in our network!?
>
> Is this behavior expected?
>
>
>
>
>
>
>
> Kind Regards,
>
>
>
> Radek Ochalski
>
>