Create a global static ARP entry
arp x.x.x.x w.w.w arpa
where x.x.x.x is the ip address of the FW and w.w.w is the MAC address. This is the only way to do this since RFC for IPv4 routers does not allow a router to accept a multicast ARP entry.
Here is an excerpt from the RFC:
RFC 1812 - Requirements for IP version 4 Routers (Section 3.3.2)
Here is the relevant part of the RFC:
3.3.2 Address Resolution Protocol - ARP
Routers that implement ARP MUST be compliant and SHOULD be unconditionally compliant with the
requirements in [INTRO:2].
The link layer MUST NOT report a Destination Unreachable error to IP solely because there is no ARP
cache entry for a destination; it SHOULD queue up to a small number of datagrams briefly while
performing the ARP request/reply sequence, and reply that the destination is unreachable to one of
the queued datagrams only when this proves fruitless.
"A router MUST not believe any ARP reply that claims that the Link Layer address of another host or
router is a broadcast or multicast address."
