IGMP Support for Network Load Balancing


Windows Server 2003 Network Load Balancing includes support for Internet Group Membership Protocol (IGMP). IGMP helps eliminate switch flooding, which may occur in a Microsoft Windows 2000-based or Microsoft Windows NT 4.0-based Network Load Balancing cluster that is running in Multicast mode.

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Earlier versions of Network Load Balancing in Multicast mode use the concept of a unicast IP address that is mapped to a multicast media access control address. Because a switch is unable to associate this multicast media access control address to a specific switch port, packets that are sent to this multicast media access control address are flooded across all switch ports, which may be undesirable in some cases. In Windows Server 2003, a Network Load Balancing cluster can also take part in a multicast group.

You must first enable IGMP snooping on the switch and you must make sure to connect each cluster node's virtual adapter to a unique switch port. When you select the IGMP option within the Network Load Balancing properties, each Network Load Balancing cluster node periodically sends an IGMP Join message that indicates the multicast group of which it is a member. Upon receiving these Join messages, the switch can associate the ports that have a Network Load Balancing cluster host attached to the multicast group. Incoming packets that are destined to the multicast media access control address are then passed only to those ports that have an attached Network Load Balancing cluster node.

The address range of the multicast group that is used is 239.255.x.y, where
x.y corresponds to the last two octets of the Network Load Balancing virtual IP address. For example, if the virtual IP address is, the multicast address that cluster nodes are a part of would be Per the RFC document 2365, this address range is administratively scoped, which means that any IGMP Join messages sent to an address in this range are not forwarded by border gateway routers. This prevents interference with any traffic on the Internet.

For additional information, click the following article number to view the article in the Microsoft Knowledge Base:
323437 HOW TO: Configure Network Load Balancing Parameters in Windows Server 2003