Fault Tolerance
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If you remove the Master, role determination will cause the Backup to take
over as the new Master. The module below the old Backup (now the new
Master) will take on the role of a new Backup. This process will repeat again if
the new Master is itself removed. Similarly, removing a Backup will cause a
new module to take over as Backup.
If you add a new Master (if you insert a new module above the current
Master), role determination will cause the current Master to relinquish
control to the new module and to take on the role of Backup (the previous
Backup, if any, would then become a Slave). Similarly, inserting a new module
between the Master and Backup (if an empty slot existed) would cause the
new module to take over as the department concentrator’s Backup.
Module Failures
The Masters and Backups frequently exchange “handshakes” in order to
verify proper operation. If the Backup does not receive a handshake from the
Master after about two seconds, it concludes that the Master has failed and
then takes over the Master’s functions.
This takeover process is not the same as the role determination process
described previously. In the case of the Master’s failure (not removal), the
Backup can take over the Master’s functions, but the chassis does not allow
another module to take over as a new Backup. Therefore, only a single level of
“hot standby” redundancy exists (in this case only).
If you were to remove the failed Master, role determination would cause the
entire department concentrator to reconfigure itself with new Master and
Backups. At that point, a new module would become the Backup to the new
Master (the previous Backup).
A failure of the Backup cannot be corrected without removing the module
itself; i.e., no other module can act as a backup for the Backup.
Configuration Data
The Master records all management agent configuration information in
nonvolatile memory. This includes:
❑
The preferred human language for display
❑
Terminal characteristics
❑
Port configurations (enabled/disabled, etc.)
❑
User-specified symbolic port, module and hub names
The Master also copies this information into the non-volatile memory of each
installed AT-3600 module. These copies are maintained in order to sustain
department concentrator reconfigurations. The management agent
remembers configuration data in spite of power failures, module failures, and
module swapouts, even of the Master itself.
