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SmartConnect User’s Guide
BMD00082, February 2009 Chapter 4: Stacking
45
Master and Backup Selection
A stack has only one Master and one Backup. The Master election is based on priority value
and MAC address. If the priority values of two Current Masters are the same, then the one with
lower MAC address becomes the Current Master for the stack. Priority is assigned internally
by how the switch is configured and its changing role in the stack, as follows:
Designated Master
A Designated Master boots up with priority base value of 150. If there is no Master for the
stack, then it is promoted to 225, the highest priority value for stacking. If the Designated
Master fails, then the Designated Backup becomes the new Master. When the Designated
Master rejoins the stack, it has priority 175 as the Current Backup.
Designated Backup
The Designated Backup boots up with priority base value of 125. When it joins a stack
with an existing Master, it becomes the Current Designated Backup with priority value of
175. If it becomes the new Master, then its priority value is 200, for the role of Current
Master.
Members
Each Member has a priority value of 100. The Designated Master (150) and Designated
Backup (125) can assume the role of Member, due to some stack event changes (for exam-
ple, merging stacks), but they still carry their base priority values.
With the above priority scheme, the Current Designated Master (225) will never be replaced by
another Designated Master (150), even if the joining Designated Master has a low MAC
address. The election process also ensures that the Current Designated Master (225) remains
the Current Master, even if another Current Master (originated from the same stack as Desig-
nated Backup with priority 200) joins the stack. The newly joined Master compares its config-
uration with the Current Master to determine if a reboot of itself is necessary.
Only the Designated Master can set the backup bit in the NVRAM of the Backup. The backup
bit can be cleared by:
By deleting or changing the Backup using the following command from the Designated
Master: /cfg/stack/backup
Another Backup is present in the same stack.
The Current Designated Master reboots with boot config set to factory default.
So the Designated Backup that replaces a failed Current Master (becomes the new Current
Master) keeps its backup bit on. This new Current Master cannot change the backup bit of
another switch, because it is not the Designated Master.