MAC address
In unicast mode MS NLB reassigns the stations MAC address (which applies to the clusters IP address) to a virtual MAC address and all NIC's in the NLB cluster use this same MAC address. This setup will cause all incoming traffic for the cluster to be flooded to all ports of the switch as unknown unicast frames: even to hosts that are not joining in the cluster. To keep flooding minimal you would need to use a dedicated VLAN for the cluster.