A Sonicwall router I don't manage is attached to a switch that I do. The router is responding to any ARP requests for addresses in 192.168.1.0/24 with it's own MAC address. IE, if I try and ping any address in that range, the ARP table is populated with an entry for that address, pointing to the MAC address of the Sonicwall. The Sonicwall does not respond to the ping, however, even though it does respond on its own legitimate address. The actual address assigned is not in that range.
The people who own it say there is nothing in the config that would cause this behavior. I thought it might be NAT or IP Helper, but they claim to have ruled these out. NAT is only setup going out from their LAN to our network, and IP Helper is disabled.
Any ideas why the Sonicwall would be responding to ARP requests for all addresses in that range? It is something of an annoyance.