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 remote IP addresses that are reachable through the fabric (EVPN).

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

E.g., for a MAC (00:02:42:42:00:20) and IP (42.42.2.20) learnt for VRF VNI 20:

    $ podman exec -it evpn-host1 ip neigh | grep br-20 | grep extern_learn | grep zebra
    42.42.2.20 dev br-20 lladdr 00:02:42:42:00:20 extern_learn NOARP proto zebra 

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

This enables ARP suppression for IPs dynamically learned through EVPN.

Notes on potential implementation details:

• ovn-controller could monitor through netlink the Linux VRF where the remote ip neigh entries are created
• OVN already supports MAC_Binding learning (for logical router ports)
• this can be extended so that externally learned MAC_Bindings are created for each logical router connected to the switch associated to the VNI where the remote MAC+IPs have been learnt