Packet Forwarding Engine Clock Generators (PCGs)

The router has two Packet Forwarding Engine Clock Generators (PCGs) installed in the slots at the rear of the chassis that are labeled PCG 0 and PCG 1, as shown in Figure 3. The PCGs generate a 125-MHz clock signal used to gate packet processing. During startup, the active Routing Engine determines which PCG is master and which is backup, and the MCS relays the decision to the PCGs and to the modules and ASICs in the Packet Forwarding Engine that use the clock signal. The modules and ASICs then use only the signal from the master source.

PCGs are hot-pluggable, as described in Field-Replaceable Units (FRUs). Removal or failure of the backup PCG does not affect router function. When 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. For PCG replacement instructions, see Replacing a PCG.

PCG Components

Each PCG (shown in Figure 7) has the following components:

• Signal generator—Provides a 125-MHz system clock signal.
• EEPROM—Stores the serial number and revision level of the PCG.
• Three LEDs—Indicate PCG status. There is a blue one labeled MASTER, a green one labeled OK, and an amber one labeled FAIL. Table 6 describes the LED states.
• Offline button—Prepares the PCG for removal from the router when pressed.

Table 6: States for PCG LEDs