Jeff here again.
In a previous blog post we covered GARPs, or GratuitousARPs. These are ARP packets that are broadcast by a client machine andentered in to other ARP caches (IP to MAC address mapping) so they instantlyknow where IP addresses are on the network. Windows XP and 2003 sent thesepackets, but Vista/2008 and Windows 7/2008 R2 did not, Windows 8/2012 also sendthese packets.
In order to make the versions of Windows more consistent, ahotfix has been released that allows Windows 7 / 2008 R2 to also send theseGratuitous ARP packets when an IP addresses is added to a NIC.
Please see this KB article for more information: Router does not update itsARP table after an IP address fails over to a cluster node in Windows Server2008 R2