Another option is to sleepmake NLB in multicast mode. The unicast [[IPv4]] address of the cluster is linked to a multicast MAC address. The hosts in the cluster will never send traffic to the switch using this MAC address with the cluster IPv4 address so one would need to create a static ARP entry on the router (layer 3) in the attached network. Not all vendors will allow you to create an ARP entry where you use a unicast IP address and a multicast MAC address. Cisco publishes some examples how to set up MS NLB on Catalyst switches running IOS<ref>Cisco: [http://www.cisco.com/en/US/products/hw/switches/ps708/products_configuration_example09186a0080a07203.shtml Catalyst configuration examples], visited 8 June 2013</ref> and these same examples can be used for switches from many other vendors. As with NLB in unicast mode: incoming traffic towards the cluster will be flooded to all ports in the switch/VLAN and not all vendors support this setup. To limit the flooding, MS NLB now supports IGMP which should lead to the switches learning which ports are actually using the multicast address,<ref>Microsoft Support knowledge base: [http://support.microsoft.com/kb/283028 KB283028: IGMP Snooping support on NLB], visited 8 June 2013</ref> but it doesn' always lead to the desired result. For example [[Dell PowerConnect]] multi-layer switches officially don't support MS NLB in multicast. Even though it does work, it will lead to high CPU utilization - affecting (other) traffic in the switch<ref>Dell Tech Communities:[http://en.community.dell.com/support-forums/network-switches/f/866/p/19435793/20091643.aspx Trying to limit switch flooding from a Microsoft Windows Network Load Balacing (NLB)], Februari 2012. Visited: 8 June 2013</ref> and on other switches one might have other limitations such that the switch to which the NLB NICs are connected can't be the same switch that does the IP routing.
