Understanding Redundant SSBs
Purpose
Inspect redundant SSBs to ensure that they provide allocation of incoming data packets throughout shared memory on the FPCs, transfer outgoing data cells to the FPCs for packet reassembly, perform route lookups using the forwarding table, monitor system components for failure and alarm conditions, and monitor FPC operation and reset.
What Are Redundant SSBs
SSBs are redundant when two SSBs are installed in the M20 router. The SSBs occupy the two top slots of the card cage (SSB0 and SSB1), and are installed into the midplane from the front of the chassis (see Figure 231). By default, SSB0 is the master SSB and SSB1 is the backup. When the master SSB fails, automatic failover occurs and the backup SSB becomes the master. You can control which SSB is the master by including the ssb statement at the [edit chassis redundancy] hierarchy level in the configuration. For more information, see the JUNOS System Basics Configuration Guide.
The SSB performs the following major functions:
- Shared memory management on the FPCs—The Distributed Buffer Manager application-specific integrated circuit (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 synchronous SRAM (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 SSB houses the Internet Processor ASIC and two Distributed Buffer Manager ASICs.
The SSB is hot-pluggable. You can remove and replace it without powering down the system; however this causes major impact to the system. The following functions cannot occur while the SSB is removed from the router:
When you remove the SSB, all packet forwarding stops immediately and the Routing Engine responds by generating alarms. When you replace the SSB, it is rebooted by flash EEPROM.
If you remove the Routing Engine, the SSB enters a warm shutdown mode and continues its forwarding process for a limited time using a frozen forwarding table. The time limit is determined by a timer in the SSB. If you replace the Routing Engine during the warm shutdown period, the SSB unfreezes its forwarding tables and resumes normal functioning. Otherwise, the SSB shuts itself down.