Packet Capture on Nexus 5K

It’s not possible to use FEX ports as a destination within a monitor session on the 5Ks, so you have to use a setup like the example below.

Note that if you’re using an Active/Active setup where your Fexes have uplinks to a pair of 5Ks, you’ll need identical source port configuration on both 5Ks and you’ll have to aggregate the desintation interfaces to your packet capture device (or capture across 2 NICs simultaneously). ERSPAN is possible, but this is the simplest solution available IMO.

interface Eth1/43
 switchport monitor
 speed 1000
 duplex full

monitor session 1
  source interface Ethernet102/1/1 both
  destination interface Ethernet1/43
  no shut