The switch fabric card (SFC) and clock scheduler card (CSC) provide the physical switch fabric for the system as well as the clocking for the Cisco cells that carry data and control packets among the line cards and route processors.On the 12000 Series, you must have at least one CSC card for the router to run. Having only one CSC card and no SFC cards is called quarter bandwidth, and only works with Engine 0 line cards. If other line cards are in the system, they will automatically be shut down. If you require line cards other than Engine 0, full bandwidth (three SFCs and one CSC) must be installed in the router. If redundancy is required, a second CSC is necessary. This redundant CSC only functions if either the CSC or an SFC goes bad. The redundant CSC can function as either the CSC or SFC.