Clustering Information on IP Address Failover

Microsoft Cluster Server (MSCS) provides the ability to define an IP address resource within a cluster, and for it to failover from one node to another.

IP address failover ability depends on two things:

• Support for dynamic registration and deregistration of IP addresses.
  
  
• Ability to update the network address translation caches of other systems attached to the subnet on which an address is registered.

Dynamic address registration and deregistration is already implemented within the Microsoft Windows NT operating system to support the lease of IP addresses using the Dynamic Host Configuration Protocol (DHCP).

Microsoft Cluster Server uses existing features within Windows NT for IP address registration and deregistration. When the cluster component attempts to bring an IP Address resource online, the software sends a command to the TCP/IP driver to register the specified address. A similar command exists to unregister an address when the corresponding resource is taken offline.

The cluster software updates the translation caches of other systems on the LAN through the Address Resolution Protocol (ARP) specification (RFC 826), which is implemented by Windows NT. The specification states that all systems receiving an ARP request must update their IP Address to physical address mapping for the source of the request (the source IP and physical network addresses are contained within the request).

Further, as part of the IP address registration process, the Windows NT TCP/IP driver broadcasts an ARP request on the appropriate LAN several times. The request asks the owner of the specified IP address to respond with its physical network address. By sending these requests for the IP address being registered, Windows NT may detect IP address conflicts; if a response is received, the address cannot be safely used.

When the driver sends these requests, Windows NT specifies the IP address being registered as the source of the request. Thus, all systems on the network will update their ARP cache entries for the specified address. Therefore, the registering system becomes the new owner of the address.

NOTE: If an address conflict occurs, the responding system may send out another ARP request for the same address, forcing the other systems on the subnet to update their caches again. Windows NT does this when it detects a conflict with an address that it has successfully registered.

For more information on the Address Resolution Protocol (ARP) as discussed in RFC 826, you may obtain a copy on the Internet from the following source: For additional information about related information, click the article number below to view the article in the Microsoft Knowledge Base: