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 10.1.2.3, the multicast address that cluster nodes are a part of would be 22.214.171.124. 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.
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