Setting bgp_next_hop in EVPN resets VNI

[Ondrej Zajicek]

On Thu, May 28, 2026 at 04:29:36PM +0200, André Grüneberg wrote:
> Hi all,
> 
> Yesterday I was partying that I was able to inject static MAC-IP routes
> into EVPN on Bird 3.3.0 with the correct VNI using the mpls_label attribute.
> 
> The joy was disturbed today, when I tried to set a different bgp_next_hop
> than the local host. As soon as I set the bgp_next_hop this forces the
> VNI/mpls_label to be encoded as 0. While the local export table shows the
> configured mpls_label, the remote side receives VNI=0.
> 
> >From reading the code, I suspect that
> https://gitlab.nic.cz/labs/bird/-/blob/v3.3.0/proto/bgp/packets.c#L1321 is
> triggering this behaviour. I have no real clue how to get values into
> nhloc->labels (statically).
> 
> I found a workaround in explicitly setting next hop address in the BGP
> protocol's evpn channel.
> 
> Yet my expectation would be that explicitly setting bgp_next_hop does not
> have any influence on treating the VNI/mpls_label.

Hi

Well, there are in fact three attributes for MPLS/VNI labels in BIRD. For one flow :

mpls_label - label for the flow on the local system

bgp_mpls_label_stack - label(s) on the bgp_next_hop system

gw_mpls - label(s) on the immediate next hop (gw) system

(unfortunately bgp_mpls_label_stack is internal and not really accessible from filters,
gw_mpls is pseudoattribute associated with the next hop, but accessible by filter)


BGP behavior is to announce MPLS label consistent with BGP next hop.

So if it uses its own address in outgoing BGP next hop, it uses
mpls_label. If it uses existing (third-party) bgp_next_hop, it uses
bgp_mpls_label_stack. If it creates BGP next hop from gw attribute, it
uses gw_mpls label.


You can see the EVPN protocol behavior when it converts ethernet route
to EVPN route (which has both mpls_label and BGP.mpls_label_stack):


bird> show route table etab all
Table etab:
4a:55:7b:73:f7:1c vlan 30 unicast [bridge1 18:37:40.738] * D (0)
    dev m32-ve1
    Type: bridge univ
    Bridge.source: dynamic


bird> show route table evpntab all
Table evpntab:
evpn mac 1:22 3 4a:55:7b:73:f7:1c * mpls 23  [evpn1 18:37:40.738] * (120)
        Type: EVPN univ
        mpls_label: 23
        BGP.next_hop: 10.1.2.1
        BGP.ext_community: (generic, 0x30c0000, 0x8) (rt, 1, 0)
        BGP.mpls_label_stack: 23


You can either use static ethernet routes and use EVPN proto to translate them,
or you can set next hop as a BGP channel option instead of as a route
property, to trick BGP thinking it is its own address and use mpls_label.


-- 
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."