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Master Selection Process
The master unit is selected from the master-enabled units (1 or 2). The factors in selecting the master unit are taken into account in the following priority:
System Up Time — The master-enabled units exchange up-time, which is measured in segments of 10 minutes. The unit with the higher number of segments is selected. If both units have the same number of time segments, and the unit ID of one of the units was set manually while the unit ID of the other was set automatically, the unit with the manually-defined unit ID is selected; otherwise the unit with the lowest unit ID is selected. If both units IDs are the same, the unit with the lowest Media Access Control (MAC) address is chosen.
Note: The up time of the backup unit is retained when it is selected as master in the switch failover process.
Unit ID — If both units have the same number of time segments, the unit with the lowest unit ID is selected.
MAC Address — If both units IDs are the same, the unit with the lowest MAC address is chosen.
Note: For a stack to operate, it must have a master unit. A master unit is defined as the active unit that assumes the master role. The stack must contain a Unit 1 and/or Unit 2 after the master selection process. Otherwise, the stack and all its units are partially shut down, not as a complete power-off, but with traffic-passing capabilities halted.
More detail here:
https://www.cisco.com/c/en/us/support/docs/smb/switches/cisco-350-series-managed-switches/smb5555-configure-stack-settings-on-a-switch-through-the-command-lin.html
Jaderson Pessoa
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