Allow resolving neighbor IP via default route

If the peer’s IP address is not in the RIB and is reachable via the default route, then you have to enable ip nht resolve-via-default. This is useful when e.g. you want to allow BGP to peer across the default route which is not so uncommon to find in production deployments.

https://docs.frrouting.org/en/latest/zebra.html#clicmd-ip-nht-resolve-via-default

https://docs.frrouting.org/en/latest/zebra.html#clicmd-ipv6-nht-resolve-via-default