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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