The Nexus 2000 as we all know gets its intelligence from its parent switch, i.e. Nexus 7000 or 5000. The Nexus 2000 for the most part is just a dumb device, otherwise known as a Fabric Extender (FEX). It may be a surprise for some, but even communication between two ports on the same fabric extender goes like this: FEX Source Port – Parent Switch – FEX Destination port. The same holds true for the Cisco UCS Fabric Interconnect (6100 and 6200 series) and the local FEX that resides in a UCS Blade Enclosure.
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.
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.
When looking at a multicast stream that has N destinations in the same blade enclosure (Cisco UCS B-Series), a few of us were thinking there would be N streams sent from the fabric interconnect to the actual FEX or blade enclosure. But, when thinking about the same example from the point of view of a Nexus 5K/2K, that would be utter non-sense – it would be the most inefficient way to distribute multicast in any data center fabric design. I think it was just the “blade” + “FEX” that was causing some confusion because the FEX is just a dumb device, right?
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.
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.
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