Addition of new switch to an existing 3850 stack

Question

I have a existing switch stack with 4 switches in the stack (all of them are 3850).

Switch Ports Model              SW Version        SW Image              Mode   
------ ----- -----              ----------        ----------            ----   
*    1 56    WS-C3850-48U       03.03.05.SE       cat3k_caa-universalk9 BUNDLE 
     2 56    WS-C3850-48U       03.03.05.SE       cat3k_caa-universalk9 BUNDLE 
     3 56    WS-C3850-48U       03.03.05.SE       cat3k_caa-universalk9 BUNDLE 
     4 56    WS-C3850-48U       03.03.05.SE       cat3k_caa-universalk9 BUNDLE 

Now, I have to add another switch (3850) to that stack.I found out that the the member number of the new switch is 1 (which is default) and the priority is set as 15. Also they new 3850 is in INSTALL mode and does not have the .bin file which is on the stack. The IOS version is the same on the stack as well as the new switch.

Here is how I am planing to add this switch:

1. Turn off the power of the new switch
2. connect the stacking cable (according to Cisco documentation)
3. Turn on the power of the new switch

By doing this the switch should automatically get the stack number and priority value of 1.

Now my question is: since the new switch has priority value of 15 before adding it to the stack, after adding the switch to the stack following above procedure, can this switch reboot the entire stack and reelect the master switch? If yes, how can I avoid this. Also will the INSTALL mode and BUNDLE mode cause any problems?

Answer 1

You should always set the switch number and priority of each switch prior to creating or adding to the stack. The switch that you want to be the master should be set to the highest priority among the switches, the one that you want to take over should be set to the next highest priority, etc. The newest switch, if you do not want it to be the master, should be set to the lowest priority.

You really want to set the priorities so that you have a deterministic method to elect the master. If you leave the priority of the new switch as 15, it will not automatically be changed to 1 as you seem to say:

By doing this the switch should automatically get the stack number and priority value of 1.

Adding a switch to an existing stack should, and probably will, work just fine, but there is always a risk that something goes wrong, so you need to be prepared to reboot the entire stack. If you want a particular switch to be the master (as you should), then you need to specifically set the switch priorities.

Answer 2

I'd recommend setting the priority of the new switch to 1. I'd also recommend converting the stack from bundle mode to install mode, which is the Cisco recommended mode of operation. As a best practice, never, ever, run a 3850 stack in bundle mode.

You're in for a reload on the stack regardless, so I'd also recommend upgrading to 3.6.6. The version of code you're on is very old and there are many improvements in later versions.

A few other things: Enable software auto-upgrade on the stack for future. As Ron mentioned, always set the preferred stack master priority to 15. I would also recommend setting the stack-mac to never expire (stack-mac persistent timer 0).

Check this link for more information.

https://www.cisco.com/c/en/us/td/docs/switches/lan/Denali_16-1/ConfigExamples_Technotes/Config_Examples/Misc/qos/m_install_vs_bundle.html