Microsoft KB Archive/244331

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MAC Address Changes for Virtual Server During a Failover with Clustering

Article ID: 244331

Article Last Modified on 10/26/2007


  • Microsoft Windows Server 2003, Enterprise Edition
  • Microsoft Windows Server 2003, Datacenter Edition (32-bit x86)
  • Microsoft Windows 2000 Advanced Server
  • Microsoft Windows 2000 Datacenter Server
  • Microsoft Windows NT Server 4.0 Enterprise Edition
  • Microsoft Cluster Server 1.1

This article was previously published under Q244331


After a failover from one cluster node to another, users in a different subnet may be unable to connect to the virtual server.


The Cluster service with Microsoft Windows NT 4.0, Enterprise Edition, and Microsoft Windows 2000 Advanced Server or Datacenter Server, and Windows 2003 Enterprise Server or Datacenter Server performs a gratuitous Address Resolution Protocol (ARP) request when a failover occurs. The gratuitous ARP request is performed to update the network (computers, routers, and switches) with the media access control (MAC) address that now owns the virtual server's IP address.

However, some devices (such as switches) may not forward the gratuitous ARP request to other devices. This causes devices on the other side of the switch or router to have the incorrect MAC address for the virtual server that has failed over. Often, this situation corrects itself after a router or switch sees the failure and updates its ARP cache by performing a broadcast. Most routers and switches are configured not to forward ARP traffic between subnets to prevent ARP storms from occurring.


Gratuitous ARP requests must be forwarded across networks so that all devices receive the updated MAC-to-IP address mappings. Contact your hardware manufacturer for information about how to change your switch or router's configuration so that gratuitous ARP requests are passed to all networks.


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.

169017 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:

199773 Behavior of Gratuitous ARP in Windows NT 4.0

168567 Clustering Information on IP Address Failover

Additional query words: MSCS vlan cisco virtualserver TCP/IP TCPIP TCP IP

Keywords: kbproductlink kbenv kbnetwork kbprb KB244331