L3VPN and BGP add-path

[Ondrej Zajicek]

On Sun, Feb 25, 2024 at 11:04:07PM +0100, Marcel Menzel via Bird-users wrote:
> Hello List,
> 
> I am running the new MPLS L3VPN feature for quite a while now, and it's
> working without issues so far for me, but I have one question:
> 
> I set "add paths on;" on my iBGP peer template in all SAFIs and while it was
> running successfully before the migration to L3VPN, the additional routes in
> the RIB are now gone, but the capabilities are still exchanged between
> peers.

Hello

I am not sure where these 'add paths on' are in your setup. On BGP
sessions ended inside VPNs? On BGP sessions between VPN routers,
with VPN SAFI or IP SAFI?

I am sure that existence of VPN SAFI should not affect functionality of
ADD-PATH on IP SAFI on the same BGP session. I did not test whether
ADD-PATH works for VPN SAFI, but i think it should.


> Furthermore, I can't set "add paths on;" property for any SAFI in the L3VPN
> protocol, this might be the reason why those additional routes are missing?

Yes, that is not supported.

L3VPN takes the best route for each IP prefix inside and convert it to
VPN route outside, and it takes best route for each (RD, IP prefix)
outside and convert it to IP route inside, so you can end with multiple
IP routes for the same prefix inside VPN, but they are due to different
RD, not due to ADD-PATH.


> My only question now is if this just hasn't been implemented yet with BIRD
> (I am just being curious hence I'm asking), my config is something missing
> or this being the general fact that add-path with MPLS L3VPN simply is not a
> thing with other vendors aswell (to be fair, I've never seen an L3VPN
> network with add-path enabled in production so far).

Well, in principle there is no reason why ADD-PATH should not work on BGP
with VPN SAFI, but they are partially overlapping features. If you want
to propagate into your internal network an IP prefix from multiple edge
routers, you need ADD-PATH to avoid best path selection in route reflector.
But if you want to propagate an IP prefix in one VPN, then each edge/PE
router would use different RD, so you do not need ADD-PATH.

Unless you already receive multiple paths to one IP prefix from CE to
your PE and want to propagate them all to other PEs, in that case
ADD-PATH would make sense.

-- 
Elen sila lumenn' omentielvo

Ondrej 'Santiago' Zajicek (email: santiago at crfreenet.org)
"To err is human -- to blame it on a computer is even more so."