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Chapter 2:
SFE2000/SFE2000P Gigabit Ethernet Switch Reference Guide
Chapter
2
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 manually-set Unit ID number are shut
down because the Stack Master cannot override the system administrator’s assignment and resolve the
conflict.
If there are more units than the maximum number allowed in a stack, and the incoming units are already
in factory default mode (they do not have unit ID assigned), then a Stack Master is elected following the
Master Discovery and Master Election processes. All other units remain shut down.