In specific deployments (e.g., with BGP EVPN configured for OVN switches/routers), ovn-controller needs the ability to monitor changes in the host FDB (mac cache) and IPv4 ARP/IPv6 ND (cache) tables.

Similar to the support for getting routing table notifications and pushing changes (route-table-notify.c/route-exchange-netlink.c) ovn-controller could use existing OVS infrastructure to:

• register itself for NETLINK_ROUTE notifications sent to the RTNLGRP_NEIGH netlink multicast group
• inject permanent (static) entries in the host FDB/neigh tables

Netlink neigh messages are of type "struct ndmsg":
https://elixir.bootlin.com/linux/v6.14.7/source/include/uapi/linux/neighbour.h#L8

struct ndmsg {
    __u8        ndm_family;
    __u8        ndm_pad1;
    __u16        ndm_pad2;
    __s32        ndm_ifindex;
    __u16        ndm_state;
    __u8        ndm_flags;
    __u8        ndm_type;
}; 

Note: ovn-controller will probably need to be able to filter notifications based on interface name.  This information is not part of the ndmsg structure; ndmsg structures only store the ifindex of the interface for which entries are updated.  That means it might be required to use/extend the OVS rtnetlink library to also monitor link updates in OVN in order to get the mapping between interfaces and ifindex. The OVS 'struct if_notifier' (lib/if-notifier.[hc]) data structure can be used to monitor interface (link) changes - therefore there's no need for any special change in OVS, it will be up to the user of the neighbor-management library to monitor and  maintain the mapping between name and ifindex.