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Today I have tested how to add an new Cisco 3850 switch into an existing stack to increase port capacity. Key command that you need to have on your stack is "software auto-upgrade enable"

My test scenario my stack is running 3.6.4, whilst my new switch has a lower firmware 3.2.3. With the command "software auto-upgrade enable" configured on my existing stack, this command will upgrade any lower firmware switch to the same version as the master. Or else you will get an output below:

sw1#show switch
Switch/Stack Mac Address : 00ca.e5b9.3400 - Local Mac Address
Mac persistency wait time: Indefinite
                                             H/W   Current
Switch#   Role    Mac Address     Priority Version  State
------------------------------------------------------------
*1       Active   00ca.e5b9.3400     1      V07     Ready            
 2       Member   346f.90b9.ea00     1      0       V-Mismatch

However if you have "software auto-upgrade enable" configured, the master switch will automatically upgrade the new switch.

sw01#
*Jul 27 19:30:03.966: %STACKMGR-1-STACK_LINK_CHANGE: 1 stack-mgr:  Stack port 1 on switch 1 is up
*Jul 27 19:30:13.605: %STACKMGR-6-SWITCH_ADDED: 1 stack-mgr:  Switch 2 has been added to the stack.
*Jul 27 19:30:36.453: %INSTALLER-6-AUTO_UPGRADE_SW_INITIATED: 1 installer:  Auto upgrade initiated for switch 2
*Jul 27 19:30:36.455: %INSTALLER-6-AUTO_UPGRADE_SW: 1 installer:  Searching stack for software to upgrade switch 2
*Jul 27 19:30:36.457: %INSTALLER-6-AUTO_UPGRADE_SW: 1 installer:  Found donor switch 1 to auto upgrade switch 2
*Jul 27 19:30:36.457: %INSTALLER-6-AUTO_UPGRADE_SW: 1 installer:  Upgrading switch 2 with software from switch 1
sw01#
*Jul 27 19:31:53.446: %INSTALLER-6-AUTO_UPGRADE_SW: 1 installer:  Finished installing software on switch 2
*Jul 27 19:31:53.446: %INSTALLER-6-AUTO_UPGRADE_SW: 1 installer:  Reloading switch 2 to complete the auto upgrade
*Jul 27 19:31:53.723: %STACKMGR-1-RELOAD_REQUEST: 1 stack-mgr:  Received reload request for switch 2, reason Auto upgrade
*Jul 27 19:31:54.118: %STACKMGR-1-STACK_LINK_CHANGE: 1 stack-mgr:  Stack port 1 on switch 1 is down
*Jul 27 19:31:54.120: %STACKMGR-6-SWITCH_REMOVED: 1 stack-mgr:  Switch 2 has been removed from the stack.
*Jul 27 19:31:54.125: Starting SWITCH-DELETE sequence, switch 2
*Jul 27 19:31:54.135: SWITCH-DELETE sequence complete, switch 2
sw01#