Jeff here again.

In a previous blog post we covered GARPs, or Gratuitous
ARPs
. These are ARP packets that are broadcast by a client machine and
entered in to other ARP caches (IP to MAC address mapping) so they instantly
know where IP addresses are on the network. Windows XP and 2003 sent these
packets, but Vista/2008 and Windows 7/2008 R2 did not, Windows 8/2012 also send
these packets.

In order to make the versions of Windows more consistent, a
hotfix has been released that allows Windows 7 / 2008 R2 to also send these
Gratuitous ARP packets when an IP addresses is added to a NIC.

Please see this KB article for more information: Router does not update its
ARP table after an IP address fails over to a cluster node in Windows Server
2008 R2