In specific deployments, e.g., with BGP EVPN configured for OVN logical switches / routers (the BGP control plane is executed outside of OVN), OVN needs to be able to send packets to MAC addresses that are reachable through the fabric (EVPN).

These remote MAC addresses are learnt by the control plane and installed in the Linux (VRF) FDB table where the control plane daemon runs.  With FRR that translates into FRR inserting externally learnt FDB entries onto the linux bridge it uses for that VRF VNI.

E.g., for a MAC (00:02:84:84:84:20) learnt for VRF VNI 20:

    $ podman exec -it evpn-host1 bridge fdb show dev vxlan-20 | grep 00:02:84:84:84 | grep extern_learn
    00:02:84:84:84:20 vlan 1 extern_learn master br-20 
    00:02:84:84:84:20 extern_learn master br-20 
    00:02:84:84:84:20 dst 20.0.0.2 self extern_learn

OVN should provide a way for users to configure an OVN logical switch to monitor and automatically install rules to forward traffic towards MAC addresses learnt by the control plane through EVPN in a given L2 VRF.

In conjunction with the support for encapsulating/decapsulating packets towards/from dynamically learned remote VTEPs from FDP-1385 this would enable stitching together L2 domains through EVPN.

Notes on potential implementation details:

• ovn-controller could monitor through netlink the Linux VRF where the remote MAC FDB entries are created
• OVN already supports FDB learning (for logical switch ports of with "unknown" addreses") and that can be extended to include MACs learnt through the netlink interface above