System Control Board (SCB)

The System Control Board (SCB) performs route lookup, filtering, and switching on incoming data packets, then directs outbound packets to the appropriate FPC for transmission to the network. It occupies the center slot of the card cage, installing into the backplane from the front of the chassis (see Figure 1).

The SCB is hot-pluggable, as described in Field-Replaceable Units (FRUs). For SCB replacement instructions, see Replace the SCB.

The SCB performs the following functions:

• Route lookups—The Internet Processor or Internet Processor II ASIC on the SCB performs route lookups using the forwarding table stored in the synchronous SRAM (SSRAM). After performing the lookup, the ASIC informs the backplane of the forwarding decision, and the backplane forwards the decision to the appropriate outgoing interface.
• Monitoring and control of router components—The SCB collects statistics from all sensors in the system. When it detects a failure or alarm condition, it sends a signal to the Routing Engine, which generates control messages or sets an alarm. The SCB also relays control messages from the Routing Engine to the router components.
• Transfer of exception and control packets—The Internet Processor or Internet Processor II ASIC passes exception packets to the microprocessor on the SCB, which processes almost all of them. The SCB sends any remaining exception packets to the Routing Engine for further processing. When the SCB detects an error originating in the Packet Forwarding Engine, it sends it to the Routing Engine using system logging (syslog) messages.
• Control of FPC resets—If the SCB detects errors in an FPC, it attempts to reset the FPC. After three unsuccessful reset attempts, the SCB takes the FPC offline and informs the Routing Engine. Other FPCs are unaffected, and system operation continues.

SCB Components

The SCB (shown in Figure 5) has the following components:

• One Internet Processor or Internet Processor II ASIC—Performs route lookups and makes routing decisions.
• Parity-protected SSRAM—Stores the forwarding table.
• Processor subsystem—Manages SCB functions and handles exception packets. The processor has the following components:

• One PowerPC 603e processor
• Parity-protected Level 2 cache
• Parity-protected DRAM

• EEPROM—Stores the serial number and revision level.
• 19.44-MHz stratum 3 reference clock—Generates clock signal for SONET/SDH PICs.
• I²C controller—Monitors the status of router components.
• Debug port—Connects the SCB to a laptop or other monitoring device through an RS-232 (EIA-232) serial cable. It uses a DB-25 connector.
• Four LEDs—Indicate SCB status. There are two green ones labeled ACTIVE and RUN, and two amber ones labeled STAT1 and STAT2. Table 4 describes the LED states.
• Reset switch—Restarts the SCB when pressed, causing the Packet Forwarding Engine to reset and halting packet forwarding for up to approximately two minutes. Do not use the reset switch under normal circumstances; to access it, push the end of a paper clip or other small probing device through the hole in the SCB faceplate.
• Extractor clips—Control the locking system that secures the SCB in the chassis.
  
  

  For specific information about SCB components (for example, the amount of SSRAM and DRAM), issue the show chassis scb command.

  
  

Figure 5: System Control Board

Table 4: States for SCB LEDs

Label	Color	State	Description
ACTIVE	Green	Flashing (pulsed with out-time proportional to traffic)	I/O interrupts are occurring.
RUN	Green	Blinking	SCB processor is running. Normally, the blinking is faint and becomes bright only when the SCB is processing many exceptions.
STAT1	Amber	Flashing	Internal diagnostics are running.
STAT2