Cisco SGE2000 Administration Manual page 24

units have been running for the same amount of time and both units have the same Unit ID,
the unit with a lower MAC (hardware) address will be selected as the Stack Master.
If the stack contains one or more units without a current Unit ID (the units are in factory
default mode), then one of these units will be the Stack Master. The unit selected to be the
Stack Master is the one running for the longest time (in intervals of 10 minutes), or, if all units
are running for the same amount of time, the one with the lowest MAC (hardware) address.
The Master Election process ensures that the stack has a Stack Master. The Stack Master has
the Unit ID of 1 and the Backup Master, if it exists, has the Unit ID of 2. Alternatively, the
Stack Master has the Unit ID of 2 and the Backup Master, if it exists, has the unit ID of 1.
If a master-enabled unit is added to a stack and powered on, when it comes up it invokes the
Master Election process, even though the rest of the stack already has an elected master.
Because the unit is new, it loses the election and joins as a stack member or Backup Master.
Unit ID Allocation and Duplicate Unit ID Conflict Resolution
After a Stack Master is elected, it allocates Unit IDs to units that do not have a currently
assigned Unit ID (units that are in factory default mode). The Stack Master also attempts to
resolve all cases of units with duplicate Unit IDs. The Stack Master changes the Unit IDs of
units that have a duplicate current Unit ID, provided that there are available, unused Unit
IDs. In a merged stack, if the Stack Master unit remains as the Stack Master, units that were
in its group will keep their unit IDs. Members of other groups are renumbered.
If the conflict occurs after the units reboot, the conflict is resolved as follows:
• If both duplicate units are in auto (self ordering) mode, then the unit ID with the
lower MAC (hardware) address will keep its unit ID. The other unit is assigned a new
unit ID.
• If one of the duplicates is in auto (self ordering) mode, and the other unit is in
manual mode, then the manual mode unit will keep its ID and the other is assigned a
new unit ID.
• If both duplicate units are in manual mode, then both of them are shut down.
If the Stack Master is able to allocate a unique Unit ID to each unit, then all units can operate
as a stack. If the Stack Master is unable to allocate a Unit ID to a unit, that unit is effectively
shut down and will not participate in the stack. For example, units with a conflicting
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Linksys One Ready Communications Solution
Chapter 3: Connecting the Switch
Understanding Advanced Stacking
3