During normal operation, both PCGs generate a 125-MHz clock signal, but only one is designated as the master. The modules and ASICs in the Packet Forwarding Engine that use the clock signal to gate packet processing use only the signal from the master PCG. For information about determining which PCG is the master, see Maintaining the PCGs.
PCGs are hot-pluggable, as described in Field-Replaceable Units (FRUs). Removal or failure of the backup PCG does not affect router function. If the master PCG fails or is removed from the chassis, however, the Packet Forwarding Engine resets so that the components start using the signal from the other PCG (which becomes the master). Packet forwarding halts while there is no clock signal, because the Packet Forwarding Engine does not accept incoming packets.
To replace a PCG, perform the following procedures: