I experienced an issue with Microsoft Network Load Balancing (NLB) configured in a multicast mode cluster. I was able to contact my cluster virtual IP address from my other computers on the same subnet (Layer 2), However I was unable to contact the Virtual IP address from a different subnet by routing through my Cisco router (Layer 3).
After a little investigation I stumbled across the following Cisco website:
To resolve this issue I was required to add the MAC address for the Virtual IP a of my NLB cluster to the Cisco router as a static MAC entry.
To grab the MAC address of the Virtual IP address first you need to make connectivity with the IP from another PC on the same subnet. Ping will do the trick. Then display the ARP cache on the PC with arp -a.
To add the static MAC address entry to the Cisco Router use the following commands.
arp 10.4.2.15 03bf.0a04.020f arpa
This will ensure you can route layer 3 to the Virtual IP of your NLB cluster when in Multicast mode.
For a detailed description as to why this happens refer to the above Cisco link.