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Network Load Balancing (NLB) - Unicast and Multicast modes

Network Load Balancing - Unicast mode

When Network Load Balancing (NLB) is operating in unicast mode, the Network Load Balancing (NLB) cluster’s Media Access Control (MAC) address is assigned to the server's Network Interface Card and the Network Interface Card's (NIC) permanently burned MAC address is disabled. All nodes in the Network Load Balancing (NLB) cluster use the Network Load Balancing (NLB) cluster’s MAC address. The incoming data packets can be received by all servers in the Network Load Balancing (NLB) cluster and passed to the Network Load Balancing driver for filtering. Network Load Balancing driver filtering ensures that only packets intended for the server are received and all other packets are discarded.

Network Load Balancing modifies the source MAC address of the outgoing packets to make it unique (to work properly with layer 2 switches). This nature of operation creates some limitation when a single Network Interface Card (NIC) server operates in unicast mode.

Network Load Balancing (NLB) is capable to work with a single Network Interface Card (NIC). There are some limitations when you configure a single Network Interface Card (NIC) in unicast mode. When you configure a single Network Interface Card (NIC) server in unicast mode, nodes within the Network Load Balancing (NLB) cluster cannot communicate with each other. However the servers can communicate with servers outside the Network Load Balancing (NLB) cluster.

Network Load Balancing - multicast mode

When Network Load Balancing (NLB) is operating in multicast mode, the Network Load Balancing (NLB) cluster's Media Access Control (MAC) address is assigned to the server's Network Interface Card (NIC) and the card's permanently burned MAC address is also maintained so that both MAC addresses can be used.

When you configure a single Network Interface Card (NIC) server in multicast mode, node-to-node communications are possible, but this mode of configuration is not optimal for handling moderate to heavy traffic from outside the cluster subnet to specific Network Load Balancing (NLB) cluster hosts.

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