Hi,
If you have matching IOS on the replacement switch as the master switch. Poweroff the replacement switch, connect stack cables and power up. The switch will boot and join the stack.
Use 'show switch' to check current master priority. Higher value wins.
"What command should I use to make sure it doesn't attempt to become the master when connecting it to switch 2?" Set priority on the replacement switch lower than the current stack master.
Tips to Add a Switch as a Slave to the Stack
To add a switch, as a slave, to a stack, complete these steps:
Note: Make sure the switch that you add into the stack has the same IOS version as the switches in the stack. Refer to to upgrade the IOS in a catalyst 3750 switch.
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Change the switch priority of the switch to be added to "1".
switch stack-member-number priority new-priority-value
Note: This step is optional, but it will make sure that the switch has fewer chances to become a stackmaster in the future.
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Power off the switch that is to be added.
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Make sure that the stack is fully connected so that, when you connect the new switch, the stack will be at least in half connectivity and does not partition.
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Connect the new switch to the stack with the StackWise ports.
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Power on the newly added switch.
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After the new switch comes up, issue the command show switch to verify stack membership.
https://www.cisco.com/c/en/us/support/docs/switches/catalyst-3750-series-switches/71925-cat3750-create-switch-stks.html#anc17
