A Nexus 2000 series Switches, often referred to as a fabric extender or FEX, relies entirely upon its parent Nexus 5000 or 7000 switch for all forwarding. Using the topology below as a reference, for host A to send a packet to host B, that packet must be sent up to the parent Nexus 5000 for a forwarding decision to be made and then back down to the FEX.
For this reason, it is critical to provide plenty of throughput to downstream FEXes; 20 Gbps for 24 GE interfaces and 40 Gbps for 48 GE interfaces are the recommended budgets. This effects a slight oversubscription ratio of 6:5, or 1.2:1. Provided that each FEX is physically located within a few racks of its parent switch, 40 Gbps can be delivered relatively cheaply using Cisco-proprietary fabric extender transceivers.