Here is the Cisco FEX Cheat Sheet on Nexus platform, Fabric Extender (FEX for short) is a companion to a Nexus 5K to 9K switch. The FEX, unlike a traditional switch, has no capability to store a forwarding table or run any control plane protocols. It relies on its parent 5K/6K/7K/9K to perform those functions. As the name implies, the FEX “extends” the fabric (ie, the network) out towards the edge devices that require network connectivity.
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