The switch fabric card contains only the switch fabric circuitry that carries the user traffic between line cards or between the RP and the line cards. The card receives scheduling information and the system clock from the clock and scheduler card. The card is keyed to occupy slots 2, 3, and 4 (lower three slots) in the lower card cage. The system can run without any switch fabric cards installed. However, if you want to maximize the bandwidth and switching capacity of the system, you must install three switch fabric cards in the lower card cage.