Cisco Nexus FEX Configuration

By | March 25, 2020

FEX ( Febric Extender ) is a brain less device and it is use to extend the ports and modules. maximum 12 FEX can be added on nexus 5k and it not supports the LACP. Because a FEX can’t generate the LACP messages. Few modules of 7K also supports the FEX. If you want to only understand the FEX configuration then you can directly see the below Cisco Nexus FEX Configuration examples:

A FEX always downloads the IOS from its parent device. A FEX has two types of ports: front panel ports and backside ports. front panel ports are use to connect with hosts or end user devices. all front panel ports are BPDU guard and portfast enable ports. backside ports are use to connect with its parent device.

FEX also not support the local switching. so all decisions will be take by the parent device. Because FEX is not having its own CPU and control plane. It also not has any console or AUX port. so we can’t access the FEX directly.

Once the FEX discovered and come online. it will appear as an additional line card on parent switch. Normally a FEX takes approximately 10 minutes to be get discover.

We can connect FEX with one or two parent switches , using one of two modes : Static pining , port channel.

Static Pining:

Pining is use to statically assign the number of links , connected to parent switch. If we are assigning 2 links and FEX is having 48 front panel ports. in this case, one link will handle 24 ports and second link will handle other 24 ports traffic. If one link fails then 24 front panel ports will be down. This situation can be overcome by configuring pining redistribute command. so these 24 ports can be move on working uplink.

N1(config)# feature fex
N1(config)# int e1/13
N1(config-if)# switchport mode fex-fabric
N1(config-if)# no shut
N1(config)# fex 100
N1(config-if)#int e1/13
N1(config-if)# fex association 100
N1(config)# fex 100
N1(config-fex)# pining max-links 2
N1# fex pining redistribute 100


Port-channel is the best option for up-links links , connected to parent switch for redundancy. It is also a kind of static pining. But we will define max-links for pining as one. because it will be consider as an single logical link.

We have to configure this type of port channel using mode ON. so port-channel can be up without any condition and negotiation.

N1(config)# feature fex
N1(config)# int e1/13 , e1/14
N1(config-if)# channel-group 12 mode on
N1(config-if)# no shut
N1(config)# fex 100
N1(config-fex)# pining max-links 2
N1(config)# int port-channel 12
           # switchport mode fex-fabric
           # fex association 101

FEX Numbers:

FEX ports don’t have any specific numbers for slot or module. so we need to configure the number for each FEX which can be set between 100 and 199.

NOTE: Two machines connected on a FEX , can’t communicate directly or without go through the parent switch. because local switching is not performed by FEX. these device can communicate only through parent switch.

Show commands:

N1# show fex 101
N1# show fex 101 detail
N1# show fex
N1# show fex 101 ver

All Below FEX topologies are supported by Nexus:

Topology type G is an exception. you can read the below note on it.

FEX Febric Ectender

Note: If G topology is configured that means , devices are supporting enhanced VPC or EVPC.

Currently, this (G) design is not supported on N7700 and N9x00.

Mix or hybrid topologies are supported in nexus like F & G. both can be configured using N7K which supports FEX but not on all models. A7K can support upto 64 FEX but number of FEX will depend on NX-OS version.

For example; NX-OS 7.3 can support up to 32 FEX as active / active. NX-OS 7.2 can support 15 active / active FEX.

But Active / Active FEX not supported woth VPC+.

Below FEX topologies are not supported by Nexus:

cisco nexus fex configuration

Thanks For Reading ………………….

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2 thoughts on “Cisco Nexus FEX Configuration

  1. Crave Freebies

    Great post. I was checking continuously this blog and I’m impressed! Extremely helpful info particularly the final phase 🙂 I handle such information a lot. I was seeking this particular information for a long time. Thank you and good luck.


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