BIRD is not prepending ASN on EBGP export.

[Alexander Zubkov]

Hi Radoslaw,

In your case, how should BIRD know in what ASN the routes are originating?
The AS number appears when you do export via eBGP session. And it uses
local ASN for that. If you need your routes to pretend to be originated in
AS20940, you need to prepend AS20940 in some filter to your routes.

On Wed, Oct 9, 2024 at 11:09 AM Ochalski, Radoslaw <rochalsk at akamai.com>
wrote:

> Thanks Ondrej,
>
> So for the routes originating in AS20940, downstream will not be aware of
> AS20940 in the as-path it will only know private AS4290006033.
>
> Kind Regards,
>
> Radek
>
>
> On 10/8/24, 5:18 PM, "Ondrej Zajicek" <santiago at crfreenet.org <mailto:
> santiago at crfreenet.org>> wrote:
>
>
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>
> On Tue, Oct 08, 2024 at 04:46:07PM +0200, Alexander Zubkov via Bird-users
> wrote:
> > 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.
>
>
> Yes, that is true. I would add that one is not supposed to have different
> local ASNs for different BGP instances that form coherent BGP router,
> unless it is configured as BGP confederation or manually patched by
> filters.
>
>
> --
> Elen sila lumenn' omentielvo
>
>
> Ondrej 'Santiago' Zajicek (email: santiago at crfreenet.org <mailto:
> santiago at crfreenet.org>)
> "To err is human -- to blame it on a computer is even more so."
>
>
>
>
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