On a user-mode server I'm running I occasionally run into the problem where a machine goes down and my ARP table shows an (incomplete) entry for the server that went down.  That incomplete arp entry does not seem to go away even if I try deleting manually with the arp -d command.
    The incomplete arp entry is on one of the tap devices (tap15) but entering 'arp -d -i tap15 ipaddr' doesn't remove it.  What's the arp command that I need to execute to delete the incomplete arp entry?