Back in November 2015 I wrote a post about FEX Topologies with the Cisco Nexus platforms, and at the time the Nexus 5K/6K line was the only model that would support the active/active FEX topology (FEX-AA), which was unfortunate in designing redundant connectivity for downstream devices.

But with the release of NX-OS code 7.2 and above, we now get FEX-AA support on the 7000 and 7700 series switches!


To recap, if you were running the 7k or the 9k switches with FEXs, you’d need single home (Straight Through) the FEX to the parent Nexus, much like the image below.  The FEX was tied to the parent switch, and we’d rely on nic teaming or multiple nics on the servers/devices connected to the FEX to provide dual homing or redundancy for connectivity.

Host vPC with FEX Straight Thru

Starting with NX-OS Release 7.2(0)D1(1), the Nexus 7000 and 7700 platforms now support FEX-AA, an active/active topology with Fabric Extenders using a vPC.  The Active/Active design allows us to connect servers/storage/devices to a single FEX, and utilize the FEXs port-channels to each parent switch to provide redundancy.  Sure, if that FEX physically fails, any devices connected will lose connectivity.

Design Requirements

The 7K Active/Active design does have a few requirements that need to be met:

  1. The only supported module for FEX-AA is the F-Series (exclusing F1 modules). The M Series modules do not support FEX-AA
  2. Straight-Through FEX and Active-Active FEX cannot exist on a same ASIC instance
  3. FEX-AA is not supported on vPC+ deployments

Configuration of the FEX-AA on the 7K follow many of the same requirements that it does on the 5K and 6K platform, such as either switch can configure the FEX and the FEX ID must be the same on both switches.

The diagram below provides the Active/Active connectivity that is now supported on the 7k.

FEX-AA Design

One topology that is still missing though from the 7K (or 9K for that matter) is what’s called the Host VPC with FEX-AA.  In the image below, we have vPC connected FEXs across the Nexus pair, and the downstream server is also connected via vPC to the FEX.  If you’re looking for physical redundancy for the servers as well, the 5K or the 6K platform would be required here.

Host vPC with FEX-AA

Moving Forward

With NX-OS 7.2, we now get another way to connect our FEXs to our parent Nexus on the 7K platform.  We still don’t have the full support for FEX connectivity that the 5K and 6K lines do, however I’m really not sure the absolutely necessity for that.

Unfortunately, the Nexus 9K platform does still not support the Active/Active topology, so for anybody using the 9K we’ll still need to use Straight Thru FEX.  Cisco’s FEX Guide for the 9K found here still points out that a dual-homed FEX is still not supported on the Nexus 9K’s.  It does call out a specific NX-OS code (7.0(3)I5(1), so here’s hoping that for the 9K’s it will be released soon!

At least Cisco acknowledges that the 9K platform has FEX-AA on the roadmap.  Great to see the different line of Nexus platforms getting features brought along to  some of the higher level switches.


The official Cisco Fabric Extender Support/Unsupported technologies can be found here.

The Nexus Fabric Extender configuration guides can be found here.

Thanks for reading!  Would love to hear  how the current supported configurations between the Fabic Extenders and Nexus switches are working for you!

2 thoughts on “Cisco Nexus 7K Design with Active/Active FEX

  1. Hello Mike, great post.
    Thinking about this FEX AA topology, I think that, at the end of the day, the most important thing is to provide the best possible redundancy for the servers (end-hosts); and this new supported topology you are losing the possibility of having Host VPC (Dual Links).

    Let´s say that you have a server with two NICs, it is possible that you want to do a NIC team (port channel), but in this scenario, you can only have the host connected to a single FEX!!…so the host availability will only depend on a single nexus 2k? I think that is still the best option to have the server connected to two n2k.

    This FEX AA topology could be useful when you only have hosts with single NIC or you want to use Active/Standby redundancy on the host side.

    Let me know your opinion, best regards.

    1. Rodrigo, you’re spot on that on the 7k you do lose the ability to have a Host vPC connection. Of course if you’re designing for downstream hosts to have a vPC connection, well you’re stuck with the FEX with a straight home connection – single point of failure on the FEX.

      I usually try to design a solution to have any Host vPC connections live on the parent switch when possible, and keep those devices that need only single home connections on the FEX. Not perfect, but it does the job.

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