Selecting Unicast or Multicast Mode Network Load Balancing can use one of two IP protocol configurations, unicast or multicast.

Network Load Balancing can use one of two IP protocol configurations, unicast or multicast. By default, Network Load Balancing will use unicast. You must decide whether the unicast mode will be suitable for your cluster. Important: The Network Load Balancing service does not support a mixed unicast and multicast environment. All cluster hosts should be either multicast or unicast or the cluster will not function properly.

Network Load Balancing’s unicast mode induces switch flooding to simultaneously deliver incoming network traffic to all of the cluster hosts. Also, when Network Load Balancing uses multicast mode, switches often flood all of the ports by default to deliver multicast traffic. However, Network Load Balancing's multicast mode gives the administrator the opportunity to limit switch flooding by configuring a virtual LAN within the switch for the ports corresponding to the cluster hosts. You can configure a virtual LAN by manually programming the switch or by using the Internet Group Management Protocol (IGMP) or the Generic Attribute Registration Protocol Multicast Registration Protocol. Note: The current version of Windows Network Load Balancing does not provide automatic support for IGMP or Generic Attribute Registration Protocol Multicast Registration Protocol.

Unicast Mode The unicast operation uses a single MAC address for all of the operations involving the cluster adapter. Using a single MAC address has the side effect of disabling all communication between cluster hosts, because outgoing packets for another cluster host are sent to the same MAC address as the sender. These packets are then looped back within the sender stack and never reach the network.

Multicast Mode Multicast provides no significant advantage over unicast from a network traffic perspective, and the increased multicast processing overhead for routers and switches may lead to degraded or slower performance. You should carefully analyze the network impact when you use multicast to avoid congestion in other network devices.

Host Communication You will need to make decisions about host communications depending on your clustering plan. If communication between hosts is required, you can use: Unicast, if multiple NICs are installed in the host. Unicast is the recommended configuration that works in both routed and nonrouted environments. The configuration complexity is greater because it has multiple NICs and usually multiple subnets. Multicast, if a single NIC is installed in the host. Multicast imposes additional restrictions within a routed environment, in that the routers must handle multicast information. Verify that any routers in your network path will support cluster multicast Address Resolution Protocol (ARP) packets. Note: It is recommended that you use Unicast with multiple NICs for maximum flexibility in a routed environment.