Microsoft NLB uses 2 modes
1. Unicast - where the actual MAC address of each server in the cluster is replaced by a common NLB Unicast MAC address. This is ok if all the servers in the NLB Cluster are connected to different switches. however, when all the servers are connected to the same switch this does not work. to solve this issue, MS has done a work around whereby a bogus MAC address per server in the cluster is created and that is assigned to the server. However the difference being this Bogus MAC is used in the Ethernet frame and not the ARP Replies. For the ARP replies to the clients, the common NLB MAC is used. Hence switch does not have an entry into the CAM table for this common NLB MAC and hence has to resort to flooding. Solution here is to have a hub in front of the NLB server cluster.
2. Multicast - In this mode, the cluster members respond to the ARP replies by sending the Cluster Multicast MAC Address (Which is illegal btw). The problem here is that the cisco switches do not accept the ARP replies where the requested IP Address is unicast but the replied MAC is multicast. hence we use the static mac entries to populate the CAM tables of the switches where the NLB cluster servers are connected.
IOS commands to add these entries are
mac-address-table static 0300.5f11.0011 vlan 10 interface fa1/1 fa1/2
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