After having a recent conversation with Cisco and a financial customer around how L2 multicast works between a UCS 6120 and a FEX, it actually uncovered something interesting with regards to how this multicast flows.
Well, let’s be honest, it is. A fabric extender is nothing, nada, zilch without its parent switch, but there is *some* intelligence when it comes to multicast replication. One stream would be sent from the parent switch to the FEX (2248, etc.) and the FEX would then replicate the packet out to all required ports, just like a L2 switch running IGMP snooping.
Here is a Cisco slide (source: Cisco LIVE 2011) that depicts the Nexus 2000 multicast forwarding architecture – this should clear up any concern one may have around multicast in a FEX architecture. The same holds true for UCS designs as well.