Linksys SLM248G4S (G5) manual Stack Units Startup Process, Master Discovery, Master Election

Appendix B About Switch Stacking

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:

1.Master Discovery/Election.

2.Unit ID allocation by the Master (including duplicate Unit ID conflict resolution)

3.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 SLM248G4S), 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:

1.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”.

2.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

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.

3.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:

24/48-Port 10/100 + 4-Port Gigabit Resilient Clustering Smart Switch with 2 Combo SFPs 52