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I am writing this to remind myself how to ADD a new switch into a live production stack without rebooting its live environment.

For this exercise, lets assume that there is an active 3 switch stack and we need to add a new switch as the 4th member.

Step 1: Physically install the new switch WITHOUT any cables connected to it.

Step 2: Power up the switch and wait for it to boot. DO NOT attached any stack cable.
Note, attaching a stack cable to a powered switch will cause the entire stack to reboot! Trust me, I found out the hard way.

Step 3: Provision the new switch. Your new switch will boot up as a standalone switch with the command switch 1 provision ws-c3850-48p.

Step 4: Renumber the switch to 4.
cmd:// switch 1 renumber 4 <cr>
save the config and reboot the switch.

Step 5: Check that the switch boots up as switch 4 after deleting switch 1 provision ws-c3850-48p

01#show switch
Switch/Stack Mac Address : 0cf5.a41c.3f00 - Local Mac Address
Mac persistency wait time: Indefinite
                                             H/W   Current
Switch#   Role    Mac Address     Priority Version  State
------------------------------------------------------------
*4       Active   0cf5.a41c.3f00     1      V04     Ready            


sw01#

Step 6: Ensure that "software auto-upgrade enable" has been configured on the main stack so that if the new switch doesn't have the same IOS firmware as the master, it will be up/downgraded.

Step 7: Unplug the switch from power; make sure it's dead. Plug the stack wise cable in.

Step 8: Power up the switch after confirming that the stack wise cable has been fully secured and screwed in.

Step 9: The 4th switch will boot and downloads its new firmware from the master if required and join the stack.