Reviewing How the Stack Master is Rebooted or Disconnected :

A new stack master is elected when you:

•Power off, disconnect, or reboot the current stack master.

•Power on all switches in a new stack at the same time.

•Reboot all switches after you manually change the stack member priorities by using the CLI.

•Connect a powered-on stack to another powered-on stack merging two separate stacks merge into one.

These rules determine switch priority:

1. You can manually set the switch priority when stack master election or re-election occurs. You can set a priority of 1 to 15; 15 is the highest priority.
This command sets switch 1 to priority 15, and it will be the stack master when you reload the stack if no other switches are also set to priority 15:

Stack(config)# switch 1 priority 15

You can then assign a lower priority to the remaining switches in the stack, or they can remain at default. 

To display information related to the stack member or the switch stack, enter the show switch user EXEC command:

Stack# show switch

Switch/Stack Mac Address : 0064.40b0.7580
H/W Current
Switch# Role Mac Address Priority Version State

———————————————————-
*1 Master 0064.40b0.7580 15 0 Ready
2 Member 0064.40a3.c480 1 0 Ready

This example shows a mixed stack of Catalyst 3750-E switches. All switches in a stack should be running the same Cisco IOS version.

Use the archive download-sw privileged EXEC command to simultaneously update the Cisco IOS image in all switches.
Reboot the upgraded switch to load and run the compatible Cisco IOS version:

Stack# reload slot 3

Enter the show switch stack-ring speed user EXEC command to see the StackWise bandwidth:

This Example shows a Catalyst 3750 stack with a complete StackWise ring.

Stack Ring Speed : 32G
Stack Ring Configuration: Full
Stack Ring Protocol : StackWise

This Example shows a Catalyst 3750 stack with an incomplete complete ring.

Stack Ring Speed : 16G
Stack Ring Configuration: Half
Stack Ring Protocol : StackWise

Use the show switch detail user EXEC command to see the status of all StackWise interfaces in the stack. The switches in this example are all active stack members, but the ring is incomplete:

show switch detail

Switch/Stack Mac Address : 0064.40b0.7580
H/W Current
Switch# Role Mac Address Priority Version State
———————————————————-
*1 Master 0064.40b0.7580 15 0 Ready
2 Member 0064.40a3.c480 1 0 Ready

Stack Port Status Neighbors
Switch# Port 1 Port 2 Port 1 Port 2
——————————————————–
1                  Ok      Ok           2          2
2                 Ok      Ok           1           1


show switch stack-ports summary

Switch#/ Stack Neighbor Cable Link Link Sync # In
Port# Port Length OK Active OK Changes Loopback
Status To LinkOK
——– —— ——– ——– —- —— —- ——— ——–
1/1 OK 2 50 cm Yes Yes Yes 2 No
1/2 OK 2 50 cm Yes Yes Yes 2 No
2/1 OK 1 50 cm Yes Yes Yes 1 No
2/2 OK 1 50 cm Yes Yes Yes 1 No

Cable Testing :

BTW – show cable-diagnostics tdr int Gi4/0/14 is very cool…

Interface Speed Local pair Pair length Remote pair Pair status
——— —– ———- —————— ———– ——————–
Gi4/0/14 1000M Pair A 79 +/- 0 meters Pair B Normal
Pair B 75 +/- 0 meters Pair A Normal
Pair C 77 +/- 0 meters Pair D Normal
Pair D 79 +/- 0 meters Pair C Normal