Proxy ARP
There are lots of requirements to get this to work.
- Router needs to see the host
on-link - Host is configured for a much larger subnet then what is actually present
- The ARP target the host is requesting is in the routing table
- The ARP target cannot be out the same interface the ARP was heard on
Lab topology
Evidences
This happens if the host is off-link for the router. The router will see the arp, but also filter it.
*May 4 20:39:31.619: IP ARP req filtered src 192.168.100.100 5254.00b7.f21b, dst 192.168.23.2 0000.0000.0000 wrong cable, interface GigabitEthernet1 tableid 0
*May 4 20:39:32.675: IP ARP req filtered src 192.168.100.100 5254.00b7.f21b, dst 192.168.23.2 0000.0000.0000 wrong cable, interface GigabitEthernet1 tableid 0
Verification
R1# show ip traffic | i proxy
0 proxy name requests, 0 where-is requests, 0 other
Sent: 0 address requests, 0 address replies (0 proxy)
0 proxy name replies, 0 where-is replies
Sent: 1 requests, 8 replies (2 proxy), 0 reverse