Dell EMC Configuration Guide for the S3100 Series 9.14.2.4

• Unit priority — User-configurable. The range is from 1 to 14. A higher value (14) means a higher priority. The default is 0. By removing the stack-unit priority using the no stack-unit priority command, you can set the priority back to the default value of zero. The unit with the highest priority is elected the master management unit; the unit with the second highest priority is elected the standby unit.
• MAC address (in case of priority tie) — The unit with the higher MAC value becomes the master unit. The stack takes the MAC address of the master unit and retains it unless it is reloaded.

To view which switch is the stack master, enter the show system command.

DellEMC#show system

Stack MAC                  : 34:17:17:18:19:1e
Reload-Type                         : normal-reload [Next boot : normal-reload]

--  Unit 1 --
Unit Type                  : Member Unit
Status                     : online
Next Boot                  : online
Required Type              : S3124F - 28-port GE/TE (S3100)
Current Type               : S3124F - 28-port GE/TE (S3100)
Master priority            : 0
Hardware Rev               : 5.0
Num Ports                  : 30
Up Time                    : 43 min, 41 sec
Dell Networking OS Version : 1-0(0-4679)
Jumbo Capable              : yes
POE Capable                : no
FIPS Mode                  : disabled
Burned In MAC              : f8:10:20:30:40:97
No Of MACs                 : 3

--  Module 1 --
Status          : not present

--  Power Supplies  --
Unit   Bay   Status       Type    FanStatus   FanSpeed(rpm)
---------------------------------------------------------------------------
  1     1    up           UNKNOWN up          0
  1     2    absent               absent      0

--  Fan  Status  --
Unit Bay   TrayStatus  Fan1    Speed   Fan2    Speed
------------------------------------------------------------------------------------
 1    1     up          up      7058    up      7164

Speed in RPM

--  Unit 2 --
Unit Type                  : Member Unit
Status                     : online
Next Boot                  : online
Required Type              : S3124 - 28-port GE/TE (S3100)
Current Type               : S3124 - 28-port GE/TE (S3100)
Master priority            : 0
Hardware Rev               : 5.0
Num Ports                  : 30
Up Time                    : 44 min, 1 sec
Dell Networking OS Version : 1-0(0-4679)
Jumbo Capable              : yes
POE Capable                : no
FIPS Mode                  : disabled
Burned In MAC              : f8:10:20:30:40:43
No Of MACs                 : 3

--  Module 1 --
Status          : not present

--  Power Supplies  --
Unit   Bay   Status       Type    FanStatus   FanSpeed(rpm)
---------------------------------------------------------------------------
  2     1    absent               absent      0
  2     2    up           AC      up          0

--  Fan  Status  --
Unit Bay   TrayStatus  Fan1    Speed   Fan2    Speed
------------------------------------------------------------------------------------
 2    1     up          up      7164    up      7058

Speed in RPM

--  Unit 3 --
Unit Type                  : Member Unit
Status                     : online
Next Boot                  : online
Required Type              : S3124 - 28-port GE/TE (S3100)
Current Type               : S3124 - 28-port GE/TE (S3100)
Master priority            : 0
Hardware Rev               : 5.0
Num Ports                  : 30
Up Time                    : 44 min, 3 sec
Dell Networking OS Version : 1-0(0-4679)
Jumbo Capable              : yes
POE Capable                : no
FIPS Mode                  : disabled
Burned In MAC              : f8:12:23:45:56:78
No Of MACs                 : 3

--  Module 1 --
Status          : not present

--  Power Supplies  --
Unit   Bay   Status       Type    FanStatus   FanSpeed(rpm)
---------------------------------------------------------------------------
  3     1    absent               absent      0
  3     2    up           AC      up          0

--  Fan  Status  --
Unit Bay   TrayStatus  Fan1    Speed   Fan2    Speed
------------------------------------------------------------------------------------
 3    1     up          up      6956    up      7058

Speed in RPM

--  Unit 4 --
Unit Type                  : Member Unit
Status                     : online
Next Boot                  : online
Required Type              : S3148P - 52-port GE/TE (S3100)
Current Type               : S3148P - 52-port GE/TE (S3100)
Master priority            : 0
Hardware Rev               : 5.0
Num Ports                  : 54
Up Time                    : 43 min, 58 sec
Dell Networking OS Version : 1-0(0-4679)
Jumbo Capable              : yes
POE Capable                : yes
FIPS Mode                  : disabled
Burned In MAC              : f8:10:20:30:40:6e
No Of MACs                 : 3

--  Module 1 --
Status          : not present

--  Power Supplies  --
Unit   Bay   Status       Type    FanStatus   FanSpeed(rpm)
---------------------------------------------------------------------------
  4     1    absent               absent      0
  4     2    up           AC      up          7920

--  Fan  Status  --
Unit Bay   TrayStatus  Fan1    Speed   Fan2    Speed
------------------------------------------------------------------------------------
 4    1     up          up      7058    up      7058

Speed in RPM
<output truncated>

When a stack reloads and all the units come up at the same time; for example, when all units boot up from flash, all units participate in the election and the master and standby are chosen based on the priority or MAC address.

When the units do not boot up at the same time, such as when some units are powered down just after reloading and powered up later to join the stack, they do not participate in the election process even though the units that boot up late may have a higher priority configured. This happens because the master and standby have already been elected, hence the unit that boots up late joins only as a member.

When an up and running standalone unit or stack is merged with another stack, based on election, the losing stack reloads and the master unit of the winning stack becomes the master of the merged stack. For more details, see sectionsAdd Units to an Existing Stack and Remove a Unit from a Stack. It is possible to reset individual units to force them to give up the management role or reload the whole stack from the CLI to ensure a fully synchronized bootup.