In a previous post I've been writing about the inability to clean the ARP cache due to cached CEF adjacencies. As it turns out, this behavior has another side effect: the router will automatically refresh all ARP entries (and CEF adjacencies) as they expire from the ARP cache. This might become a problem on high-end devices with a lot of directly connected hosts if you set the arp timeout to a low value.

Here is a sample debugging printout verifying this behavior:

Jun 7 16:34:49: IP ARP: sent req src 192.168.0.1 0016.c7fe.f150,dst 192.168.0.2 000c.293a.b455 FastEthernet0/0.100
Jun 7 16:34:49: IP ARP: rcvd rep src 192.168.0.2 000c.293a.b455, dst 192.168.0.1 FastEthernet0/0.100
Jun 7 16:34:49: IP ARP: creating entry for IP address: 192.168.0.2, hw: 000c.293a.b455