Administration: Stack Management
Unit Failure in Stack
Cisco 350, 350X and 550X Series Managed Switches, Firmware Release 2.4, ver 0.4 126
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When a master is being configured, it synchronizes the backup immediately. Synchronization
is performed as soon as a command is executed. This is transparent.
If a unit is inserted into a running stack, and is selected as a backup unit, the master
synchronizes it so that it has an up-to date configuration, and then generates a SYNC
COMPLETE SYSLOG message. This is a unique SYSLOG message that appears only when
backup is converging with the master unit, and looks like this: %DSYNCH-I-
SYNCH_SUCCEEDED: Synchronization with unit 2 is finished successfully.
Master/Backup Switchover
When a master fails on the stack, a switchover occurs.
The backup unit becomes the master, and all of its processes and protocol stacks are initialized
to take responsibility for the entire stack. As a result, there is temporarily no traffic forwarding
in this unit, but slave units remain active.
NOTE When STP is used and the ports are in link up, the STP port’s state is temporarily Blocking, and
it cannot forward traffic or learn MAC addresses. This is to prevent spanning tree loops between
active units.
Slave Unit Handling
While the backup becomes the master, the active slave units remain active and continue to
forward packets based on the configuration from the original master. This minimizes data
traffic interruption in units.
After the backup unit has completed the transition to the master state, it initializes the slave
units one at a time by performing the following operations:
• Clear and reset the configuration of the slave unit to default (to prevent an incorrect
configuration from the new master unit). As a result, there is no traffic forwarding on
the slave unit.
• Apply related user configurations to the slave unit.
• Exchange dynamic information such as port STP state, dynamic MAC addresses, and
link up/down status between the new master and the slave unit. Packet forwarding on
the slave unit resumes after the state of its ports are set to forwarding by the master
according to STP.
NOTE Packet flooding to unknown Unicast MAC addresses occurs until the MAC
addresses are learned or relearned.
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