BIRD is not prepending ASN on EBGP export.

[Ochalski, Radoslaw]

We receive this route via IBGP on IBPG session and then exporting it to EBGP, I am pretty sure most vendor implementations will automatically prepend IBGP ASN. Otherwise, we are missing one ASN in ASPATH and attracting more traffic.

Yes, I am aware we can prepend it, though it seems strange this is not default behavior.

Kind Regards,

Radek


From: Alexander Zubkov <green at qrator.net>
Date: Wednesday, October 9, 2024 at 3:30 PM
To: "Ochalski, Radoslaw" <rochalsk at akamai.com>
Cc: Ondrej Zajicek <santiago at crfreenet.org>, Bird-users <bird-users at network.cz>
Subject: Re: BIRD is not prepending ASN on EBGP export.

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,
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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<mailto: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> <mailto: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> <mailto: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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