Extreme Switches and Microsoft Multicast NLB
I’ve recently been troubleshooting an issue with Extreme Switches and Microsoft Multicast NLB. The Extreme switches (along with many other major Layer 3 switch vendors – Cisco, Juniper etc) have the issue that they break the NLB’s ability to mask the MAC address in use.
When you use Extreme Switches and Microsoft Multicast NLB, each cluster host retains the original MAC address of the adapter. In addition to the original MAC address of the adapter, the adapter is assigned a multicast MAC address, which is shared by all cluster hosts. The incoming client requests are sent to all cluster hosts by using the multicast MAC address. From what I could see on the Extreme switches the Multicast MAC never entered the FDB table.