A good portion of the Dell switches support NLB in Unicast mode only, and NOT in Multicast mode. The Dell Networking Force10 switches typically support NLB Multicast mode. [URL] Hope this info helps. 
Our Cisco SG300-10 switch is incompatible with windows multicast NLB because it does not allow to add multicast 0100.5e7f.c801 or 03bf.0ada.c801 MAC addresses to ARP Does Dell PowerConnect 2824 (and 2808) work with windows multicast NLB (IGMP multicast preferably)?  
Windows Server IGMP multicast NLB support on PowerConnect 2824