My Cisco ACI FEX Study Notes

ALE: the leaf ports that connect to the spine

If we connect a 40G FEX to a 10G leaf, we need to use Breakout cables, which are cables with a QSFP on a one side and 04 10G SFP on the other side. And it is a Cisco cable.

A FEX connects only to leafs. There is no dual-homed FEXes; only normal port channels to a parent switch. Between the FEXes we can configure a vPC and apply the same VPC Interface Policy Group to both FEXes. On the FEX user-facing ports, we can attach a server through vPC:

Connectivity between FEX and Leaf © Cisco.com

We must distinguish the FEX Switch Profile from the normal Switch Profiles.

To indicate in a Leaf that we are connecting a FEX, we activate the checkbox “connected to a FEX” under the Access Port Selector of the parent switch (i.e. the leaf).

When attaching and booting FEXs to ACI fabric, it downloads its code from APIC. It can take some time to appear with the following command:

Leaf# show fex

For the FEX we must create:

  • a dedicated AAEP,
  • a VLAN Pool as static assignment,
  • a physical domain,
  • an interface policy group.

We need to configure two different Switch Profiles: one as a FEX Switch Profile and one as a Leaf Switch Profile for the parent switch’s.

We must distinguish the FEX Interface Profile from the parent switch’s Interface Profile.

The FEX Interface Access Selector is configured under the FEX Interface Profile.

If you see “out-of-service” under a FEX access interface (i.e. user-facing interface) in the output of:

Leaf# show interface status

then you know that this interface is not yet associated to an EPG as part of Static Path Binding.

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