FEX vs. Fabric extender

[TomElgin49285]

I'm having some difficulty understand the difference between a FEX and the Fabric Interconnect (other than the FI contains the UCSM and allows you to configure servers via a GUI and the FEX being essential a virtual line card in a NxK switch). I understand B-series blade servers must be plugged into the FI.

Is the FI a card in a chassis?

Why use one over the other?

Why not just use the FI all the time if it has the UCSM????

[Hector Gustavo Serrano Gutierrez]

Hi @TomElgin49285,

Is the FI a card in a chassis?

The form factor of a FI is like any Switch.

For instance, this is a Cisco UCS 6332-16UP Fabric Interconnect.

Why use one over the other?

You use an FI when there is at least one UCS B-series chassis since you must manage the UCS B-series chassises and its blade servers using UCSM which is housed by the FI.

The word "FEX" can actually be used to describe different escenarios. Generally, the FEX denotes that you are able to configure a switch interface which is not physically part of the "parent Switch" where you are performing the configuration.

You can for instance have your FI located at the end of the row (EoR) in your Data Centers and the FEX at the top of the rack (ToR). The benefit of this is that a) you gain more ports and fewer cabling (still you need to watch out for bandwidth requirements) b) you still manage all the FEX switches by just issuing configuration only on the "parent Switch" and not on every single ToR switch.

Refer to the following picture. Remember that even though UCSM is mandatory to manage UCS B-series chassis, you can optionally use it to manage your UCS C-series rack servers as well using the same pair of FIs (most common scenario) in a single UCS domain.

Why not just use the FI all the time if it has the UCSM????

Yes, you can do that but with FEX switches you have more options available in case the business needs it ;-)

I hope this helps.

[TomElgin49285]

Thanks for that explanation Hector. Since posting this, I also found out that the term "fabric extender" cannot only apply to the Nexus 2000 (which is essentially a line card, as you mentioned), but also to the IOM/FEX in

the blade server chassis (as it is an extension of the fabric interconnect), and the following as well:

• Adapter FEX, which splits a physical NIC into multiple logical NICs. This is done using UCS and a mezzanine card on the blade server. This can also be used to create virtual HBAs.
  
  
• VM FEX extends Adapter FEX technology to VMs. In this case, you take the adapter FEX, split it into multiple NICs (or HBAs), and "insert" those into virtual machines. In effect, you're extending the hardware into the VMs. This allows you to manage VM NICs from UCS!

So much new terminology and concepts to learn in the DC environment. Yikes!

[Hector Gustavo Serrano Gutierrez]

That is correct @TomElgin49285,

As a summary:

1. FEX Switch (Nexus 2000 family of Switches which connect to Nexus 7000 or 9000 acting as parent Switches).

2. IOM FEX (which goes in the UCS B-series chassis and connects to the FI as parent Switch).

3. Adapter FEX.

4. VM FEX.

Cheers!