Cisco WS-X4993-F Catalyst 4948E Fan Tray with Four variable-speed 12V DC Fans; Back-to-Front Cooling; Mounted in the rear of the Catalyst 4948E Switch Chassis between the two Power Supplies; Supported only on the Cisco Catalyst 4948E-F Switch; Front Panel is color-coded Blue and keyed to prevent insertion in the Catalyst 4948E Chassis.