Fabric Extenders (FEX) modules are also known as IO Modules (IOMs). These modules serve as line cards to the Fabric Interconnects, in the same way as the Nexus series switches' remote line cards. IOMs also provide interface connections to blade servers. IOMs multiplex data from blade servers and provide this data to Fabric Interconnects and do the same in the reverse direction. In production environments, IOMs are always used in pairs to provide redundancy and failover.

Apart from data transfer between chassis and Fabric Interconnects, IOMs provide two other functions:

• Chassis Management Controller (CMC): This monitors chassis components, such as fan units, power supplies, and chassis temperature. It also reads chassis components' identification data and detects the insertion and removal of blade servers.
• Chassis Management Switch (CMS): This provides fast Ethernet links to embedded Cisco Integrated Management Controller (CIMC) instances on blade servers for KVM access, serial over LAN (SoL) access, and Intelligent Platform Management Interface (IPMI) data to travel from individual blades to FIs for monitoring and management purposes.

The following is a diagram of the Cisco UCS Fabric Extender (IO Module):