Understanding the SCB
Purpose
Inspect the SCB, the control board for the Packet Forwarding Engine, to ensure that it provides route lookups, system component monitoring, exception and control packet transfer, and FPC operation and reset control.
What Is the SCB
The SCB is the control board for the M40 router Packet Forwarding Engine (see Figure 160).
The SCB performs four major functions:
- Route lookups—The Internet Processor application-specific integrated circuit (ASIC) on the SCB performs route lookups using the forwarding table stored in synchronous SRAM (SSRAM). After performing the lookup, the Internet Processor informs the backplane of the forwarding decision, and the backplane forwards the decision to the appropriate outgoing interface.
- System component monitoring—The SCB monitors other system components for failure and alarm conditions. It collects statistics from all sensors in the system and relays them to the Routing Engine, which sets the appropriate alarm.
- Exception and control packet transfer—The Internet Processor ASIC on the SCB passes exception packets to a microprocessor on the SCB, which processes almost all of them. The remaining packets are sent to the Routing Engine for further processing. Any errors originating in the Packet Forwarding Engine and detected by the SCB are sent to the Routing Engine using
SYSLOGmessages. - FPC reset control—The SCB monitors the operation of the FPCs. If it detects errors in an FPC, the SCB attempts to reset the FPC. After three unsuccessful resets, the SCB takes the FPC offline and informs the Routing Engine. Other FPCs are unaffected, and normal system operation continues.
The SCB occupies the center slot of the card cage, and is installed into the backplane from the front of the chassis (see Figure 161).
The SCB is field-replaceable and hot-pluggable. You can remove and replace it without powering down the system; however, this causes major impact to the system. While the SCB is out of the router, route lookups, system component monitoring, exception and control packet transfer, and FPC operation and reset control cannot occur.