JUNOS Internet Software Network Operations Guide: Hardware
394 ! Understanding the SCB
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).
Figure 160: SCB Component
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
SYSLOG messages.
! 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.
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