What issues can occur upgrading my 3850 IOS from 03.06.05E to 16.03.05 and adding a switch to a stack?

I have a stack consisting on 4 WS-C3850-48 switches...
2 WS-C3850-48T CAT3K_CAA-UNIVERSALK9, Version 03.06.05E IPBase - INSTALL mode
2 WS-C3850-48P CAT3K_CAA-UNIVERSALK9, Version 03.06.05E IPBase - INSTALL mode

I want to add a 10 GB switch to the stack.  It is a...
WS-C3850-12XS-S CAT3K_CAA-UNIVERSALK9, Version 16.03.05 IPBase - INSTALL mode

I am wanting to use the basic Cisco documents about upgrading my current stack to 16.03.05 from 03.06.05E.  Are there any special considerations?  I am only a CCENT with limited experience.  I don't want to ruin my stack.

I am backing up my configs and VTP database.

I believe the switch can be added while the stack is on.  Is this true?  What order would I unplug the stack cables?
Since it is a stack, will each switch take turns upgrading and avoid any downtime?
Mark HaggittNetwork AdministratorAsked:
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atlas_shudderedSr. Network EngineerCommented:
There are a few things that you need to confirm and configure prior to installing the new switch.  Also, it is best if you perform this process during a maintenance window as reboots may be necessary and there is a risk of hardware failure.

1.  Pull current running and startup configs from existing stack.
2.  Confirm the switch priorities in the current stack.  Make sure that your master switch is set to priority 15 and all other current switches are set to a decremented priority less than 15 ( I generally decrease each additional switch by 2 - e.g. Master 15, Switch 2 - 13, Switch 3 - 11, etc.)

conf t
switch X priority Y
(example: switch 1 priority 15)

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3.  Power on the new switch apart from the stack and confirm the IOS version and package installed to the switch matches that of the stack.  If it does not, load the appropriate IOS to the switch and confirm that it boots to that image.  Don't rely on the stack to perform smart upgrade.  It is always a crap shoot and generally works out badly for the installer.  In your case you are going to want to upgrade the existing stack and this is where things will get a bit tricky.  I prefer to do the upgrade one switch at a time.
4.  Bring the master and all slaves down, keeping one slave powered on.  Connect to the switch via console and ethernet and tftp your upgrade file to the switch (insure that the port you are connecting your PC to is in the vlan of the SVI of your management interface for that switch.
5.  Upon completing upload, reboot the switch and issue
show ver

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at the exec prompt to confirm the new IOS has taken and is running.
6.  Once confirmed, bring the switch back down and move on to the next slave in the stack.  Rinse and repeat steps 4 and 5 until you have upgraded all existing stack switches with the original master being upgraded last.
7.  Bring the master back online.  Once the master is online, bring the rest of the stack back online.
8.  Configure the new switch's stack priority.  Same configuration as above in 2.
9.  Rack the new switch and power it on.
10.  Once the new switch has completed post and is up, cable the new switch into the stack via smartstack cabling.  I generally connect the new switch to the A side of the stack ring first but I don't think that it really makes a difference.

It will take 5-10 minutes for the new switch to be recognized and be introduced to the existing stack.  If memory serves the new switch will go through 1-2 reboots during this period.

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Mark HaggittNetwork AdministratorAuthor Commented:
Thank you very much.  This is very helpful and complete.  I have a question about my current stack.   All of the switches have a priority of 1.  The second switch in the stack is acting as master.  Should I keep the second switch as the master when I set the priorities, or do I need to figure out which switch was intended to be the master?  Is there any feasible reason the original installer would have set all the switches to have a priority of one?  Is it likely that it was an oversight and/or lack of knowledge?
Mark HaggittNetwork AdministratorAuthor Commented:
Here is a question from my boss has.  If a switch comes with an IOS in the version 16 family, is it capable of being downgraded to the version 3 family?  We understand version 16 is intended to be forward facing tech.
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atlas_shudderedSr. Network EngineerCommented:
To your first question, I would say yes, keep the second as master.  And in the future, when you place a switch into the network, change it's priority regardless of whether it is standalone or stacked.

To the second question - depending on the type/model switch, yes you can roll them back but.....

Before you attempt you need to check cisco's website for compatibility and any known issues with the version of IOS you are planning to use, regardless of upgrade or downgrade of the code.
Mark HaggittNetwork AdministratorAuthor Commented:
It looks like I will not be able to downgrade to version 3.6.5E.  Maybe, I can downgrade to 3.7.5E.
Do all the switches in a stack necessarily have be the same version?  Can one switch be version 3.7 while the others are 3.6?

We are thinking if we can downgrade the new switch, we can save downtime.
atlas_shudderedSr. Network EngineerCommented:
Yes.  All switches in a stack have to be both same version and same license level, e.g. lanbase, ipbase, etc.
Mark HaggittNetwork AdministratorAuthor Commented:
Thank you!
atlas_shudderedSr. Network EngineerCommented:
No worries chief.  Glad to help.  Hopefully the mods let me keep my points on this one!
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