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New Member

FEX numbering

We have two Nexus 5020's that will be deployed in a data center with several FEX . I haven't decided yet whether to single or dual home the FEX's or if I'm going to use vPC's right away.

Are there things I should consider when numbering the FEX's ? If I don't use vPC's then I could duplicate FEX numbers across both 5020's, but is that going to cause problems later if I use vPC's ? Do most people keep the FEX numbers unique between 5020's ? (ex. odd on one and even on the other).

Any thoughts on the subject would be appreciated.


New Member

Re: FEX numbering

As I understand it, if you want to do vPC down to the host from the FEX, the FEX can not be multihomed.

Someone correct me if I am wrong, the FEX is in essence a line card hanging off the N5K. If you multihome you can't make the FEX themselves a vPC domain, so you can't take advantage of port channeling/multihoming up from the host which is a superior configuration.

Its new to me to though, I could be mistaken.

New Member

Re: FEX numbering

That's my understanding also, you can only do port channels to the host when you use a single connection from two different single homed FEX's (using vPC).

Back to my original question, for those with multiple 5020's and FEX's on each one, are you keeping the FEX numbers unique ? and does it matter if the 5020's are vPC peers ?

New Member

Re: FEX numbering

In case anyone else was wondering about this, I asked my SE about it.

He says that you should keep your FEX's numbered differently between different 50xx's, if you do end up dual homing the FEX's you will have problems if they are the same number.

New Member

Re: FEX numbering


Contact me off-line at, I am trying to setup the similar env here and, are facing this dilemma as well. Leo

New Member

Re: FEX numbering

I am not so sure of the statement your Cisco SE made.

Actually the FEX numbers (1 FEX dual-home to 2 NX5K) must be same, otherwise, it won't associate them successfully, and "show fex xxx detail" will complain "... .... State. Identity-Mismatch".

Also, you have to configure same FEX host port (host port, not teaming, no port channel whatsoever, just a plain host port) TWICE on both NX5K switches BEFORE you can use the port, if you do not do that, the FEX port won't show up, and the syslog would complain "...are being suspended. (Reason: Vlan is not configured on remote

vPC interface)"

Hope it helps.

I am going to vPC our 2 NX5K to C6500s via multiple GEs, if everyone has successfully done that, feel free to jump in.

apparently, the Cisco Documents and TAC are way behind their production line pace.


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