Home » Exchange Server 2010 » Client Connectivity » Methods for Configuring NLB

Methods for Configuring NLB

Configuring NLB (Network Load Balancing)

NLB can be configured either in unicast or multicast mode.


What is Unicast & MultiCase?


Unicast :


In Unicast mode, NLB reassigns the station MAC (media access control) address of the network adapter for which it is enabled and all cluster hosts are assigned the same MAC address

Unicast mode induces switch flooding, where all switch ports are flooded with NLB traffic, even ports to which non-NLB servers are attached.

Since all hosts in the cluster have the same IP Address and the same MAC Address, there is no inter-host communication possible between the hosts configured in Unicast mode. A second NIC is therefore needed for other host communication.




In multicast mode, NLB assigns a layer-2 multicast address to the cluster adapter instead of changing the adapter’s station address.


Multicast allows inter-host communication because it adds a layer two multicast address to the cluster instead of changing it. This makes inter-host communication possible as the hosts retain their original unique MAC addresses and already have unique dedicated IP addresses.


However, in multicast mode, the ARP reply sent out by a host in the cluster in response to an ARP request, maps the cluster’s Unicast IP Address to its multicast MAC Address. Such a mapping in an ARP reply is rejected by some routers, so administrators must add a static ARP entry in the router mapping the Cluster IP Address to its MAC Address.


For More Info on Unicast & Multicast:  Unicast & Multicast (Technet)

Unicast is the preferred method for NLB on Hyper-V


Praveen Kumar

MCTS, MCITP | Exchange Server

Publisher @


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