In a cluster, each computer (or cluster node) has a network adapter attached to the corporate network, and each cluster node has its own IP address, network name (NetBIOS name), and MAC address. The virtual server has an IP address and network name, but uses the MAC address of the cluster node that is the current owner of the virtual server resources.
Information on Groups and Resources Using Microsoft Cluster Server
When a failover occurs, the cluster server of the node that is receiving IP resources sends a gratuitous ARP request so that all devices (computers, routers, and switches) are updated, and a new MAC address is assigned to an existing IP address. If a switch or router does not pass the updated MAC-to-IP address mappings, other network devices contain the old MAC address for the cluster node that is down.For additional information, click the article numbers below to view the articles in the Microsoft Knowledge Base:
Behavior of Gratuitous ARP in Windows NT 4.0
Clustering Information on IP Address Failover