The heart of the Cisco Internet Router is a crossbar switch fabric that provides synchronized gigabit-speed interconnections between the line cards and the route processor (RP). The switch fabric for the router consists of two clock and scheduler cards (CSCs) and three switch fabric cards (SFCs) installed in the switch fabric card cage.The clock and scheduler card provides the following functionality. The switch fabric carries the user traffic between line cards or between the RP and a line card. The switch fabric on the CSC is identical to the switch fabric on the SFC.The scheduler handles requests from the line cards for access to the switch fabric and determines when to allow the line cards access to the switch fabric.The system clock synchronizes data transfers between line cards or between line cards and the RP through the switch fabric.