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Redundant SSBs on the M20 Router
The System and Switch Board (SSB) on the M20 router performs
the following major functions:
- Shared memory management on the FPCs—The Distributed
Buffer Manager ASIC on the SSB uniformly allocates incoming data packets
throughout shared memory on the FPCs.
- Outgoing data cell transfer to the FPCs—A second
Distributed Buffer Manager ASIC on the SSB passes data cells to the
FPCs for packet reassembly when the data is ready to be transmitted.
- Route lookups—The Internet Processor ASIC on the
SSB performs route lookups using the forwarding table stored in SSRAM.
After performing the lookup, the Internet Processor ASIC informs the
midplane of the forwarding decision, and the midplane forwards the
decision to the appropriate outgoing interface.
- System component monitoring—The SSB 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. For example, if a temperature sensor
exceeds the first internally defined threshold, the Routing Engine
issues a “high temp” alarm. If the sensor exceeds the
second threshold, the Routing Engine initiates a system shutdown.
- Exception and control packet transfer—The Internet
Processor ASIC passes exception packets to a microprocessor on the
SSB, which processes almost all of them. The remaining packets are
sent to the Routing Engine for further processing. Any errors that
originate in the Packet Forwarding Engine and are detected by the
SSB are sent to the Routing Engine using system log messages.
- FPC reset control—The SSB monitors the operation
of the FPCs. If it detects errors in an FPC, the SSB attempts to reset
the FPC. After three unsuccessful resets, the SSB takes the FPC offline
and informs the Routing Engine. Other FPCs are unaffected, and normal
system operation continues.
The M20 router holds up to two SSBs. One SSB is configured to
act as the master and the other is configured to serve as a backup
in case the master fails. You can initiate a manual switchover by
issuing the request chassis ssb master switch command. For
more information, see the JUNOS System Basics and Services Command Reference.
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