The Gratuitous ARP.
We currently face an issue where everyone is throwing around the term, ‘Gratuitous ARP.’ It is a problem that is causing us issue with rolling database instances to new hardware, or another example would be moving IP addresses in a cluster scenario.
The real issue faced is that of the behavior involving Windows Server 2008 R2.
First, a Windows Vista or Windows Server 2008 will not update the Neighbor cache if an ARP broadcast is received unless it is part of a broadcast ARP request for the receiver. What this means is that when a gratuitous ARP is sent on a network with Windows Vista and Widows Server 2008, these systems will not update their cache with incorrect information if there is an IP address conflict.
…more to come after I sleep. An extra long day in which many things were broken.