The status arp proxy server or IP Forwarding Agent/IP Virtual Router
are informational flags; the heuristics for setting them are open for
modification or even removal. The "IP Forwarding Agent" flag is set for
a MAC address when, for that MAC address, two different IP addresses are
seen in direct succession, without an intervening 0.0.0.0 address (i.e.
without a DHCP request).
A MAC address is flagged as an IP Virtual Router when it is already
flagged as an IP Forwarding Agent, and, in addition to that, the AP
witnesses it responding to ARP requests with two different IP addresses
(for the same MAC address).