We are currently deploying new data centers using brand new Cisco Nexus 5596UP switches with Fabric Extenders (Cisco Nexus 2248TP-E). When you order a bundle from Cisco (switches and FEXes) it goes pre-packed with FET-10G modules (8 per FEX). Ok, not pre-packed, you actually need to include those modules into the order for free. The reason I am making notes on this is due to the additional configuration required for this module to be discovered. If you simply put it into the switch, it will show those modules as Invalidated. For them to work, it is required to manually change switchport mode to “fex-fabric” – switchport mode fex-fabric. Once you’ve done that, FET-10G will be recognized and any FEX behind it will be discovered.
This can be confusing if you’re deploying Nexus switches and FEXes using this modules for the first time. They have a limited scope of use – only switch to FEX connections. They go for free when you buy bundles, so make sure to include them into your order! You won’t be able to get them for free later (or even buy them). General SFP+ modules will discover any connected FEXes automatically.