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About Switch Stacking
24/48-Port 10/100 + 4-Port Gigabit Smart Switch with Resilient Clustering Technology and PoE
Appendix B
Stack Units Startup Process
Whenever a unit in stack mode is initialized (powered
up or rebooted) it goes through the same exact process,
consisting of the following three steps:
Master Discovery/Election.
Unit ID allocation by the Master (including duplicate
Unit ID conflict resolution)
Unit/port configuration by the Master
Master Discovery
Whenever a unit in stack mode initializes, its behavior will
depend on its Unit ID (if any).
If the unit does not have a current Unit ID (that is, the
unit is in factory default mode).
If there is a master, the unit performs Unit ID Allocation
(refer to section “Unit ID Allocation and Duplicate Unit
ID Conflict Resolution”), where it will get a number from
the master. If there is no master, then it will participate
in Master-election, and may even end up as the new
master or backup master.
If the unit’s current Unit ID is 1 or 2 (previously allocated,
perhaps even in a different stack), then the unit will
participate in the Master election.
If the unit has a current Unit ID (previously allocated,
perhaps even in a different stack), it will try to use this
number in the new stack. If the unit’s current Unit ID is
3-6 (3-4 for SLM248G4PS), then it will try and connect
to the running stack Master, and will not proceed to
the next stage until contact with the Master is made. In
particular, such units will NOT participate in the Master
election process, and if no Master is present, the units
will be effectively shut down.
Both the master and all other stack units carry out a
continuous process of master discovery by frequently
exchanging stack control messages. This allows them to
discover whenever a unit fails or becomes unreachable.
Master Election
Whenever a unit (or more than one) in stack mode comes
up, one of the units is elected to be the stack Master. The
unit selected as Stack Master is chosen as follows:
If a unit in the stack was set to “Force Master” by the
system administrator, that unit will be the stack Master.
Only master enabled stack units, i.e. unit ID=1 or unit
ID=2 can be selected as “Force Master”.
Otherwise, if the stack contains units whose unique
Unit ID is either 1 or 2, then one of these two units
will be the stack Master. It does not matter if the Unit
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ID was originally assigned automatically or manually.
(Such units are called Master-enabled units).
If there is only one such unit, it will be selected as
the stack Master (even if its Unit ID=2).
If there are two such units, the two units will decide
which of them is the Master by checking:
Which one has been running for a longer time
(in increments of 10 minutes). The unit running
for a longer time will be the stack Master.
If they have been running for the same amount
of time, Unit ID=1 will be the stack Master.
If both units have been running for the same
amount of time and both units have the same
Unit ID, the unit with a lower MAC address will
be selected as stack Master.
Otherwise, if the stack contains one or more units
without a current Unit ID (that is to say in factory
default state), then the stack Master will be one of
these units. The unit selected to be the Master will be
the one running for the longest time (in increments
of 10 minutes) or, if all units are running for the
same amount of time, the one with the lowest MAC
address.
The end result of Master Election is that the stack has a
stack Master. The Stack master has unit ID=1 and the
Backup Master, if it exists, unit ID=2. Alternatively, the
Stack master has unit ID=2 and the Backup Master, if it
exists, unit ID=1.
If a Master-enabled (Unit ID=1 or 2) unit is added to a
stack and turned on, then when it comes up, it will invoke
master-election, even though the rest of the stack already
has an elected master. Because it is new, it will lose the
election and join as a member or backup master.
Unit ID Allocation and Duplicate Unit ID Conflict Resolution
Once a stack Master is elected, it will allocate Unit IDs to
units that do not have a currently assigned Unit ID (that is,
units in factory default mode).
In addition, the stack Master will try to resolve all cases of
units with duplicate Unit IDs. This is done by changing the
Unit IDs of offending units that have a duplicate current
Unit ID, provided that there are available, unused Unit
IDs.
In the case of a merged stack scenario, units that were
initially in the sub-group of the Master that remained as
Master will have the same unit IDs as they had before.
Members of the other sub-group will be renumbered.
If the conflict occurs after the units reboot then the
following will take place:
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