Hello Simon
I looked over some of the release notes of the 17.12.X version. There is no explicit explanation of the boot process for a switch stack, but the upgrade process does seem to indicate that the install add file command will automatically reboot the primary switch, and will wait for all other switches in the stack to reboot as well:
Preparing to autoboot. [Press Ctrl-C to interrupt] 0 boot: attempting to boot from [flash:packages.conf] boot: reading file packages.conf ######################################################################################################################################### ######################################################################################################################################### Waiting for 120 seconds for other switches to boot Switch number is 1 All switches in the stack have been discovered. Accelerating discovery <output truncated>Note: The system reloads automatically after executing the install add file activate commit command. You do not have to manually reload the system.
This is relatively conclusive proof that the reboot takes place for all devices at the same time and there’s no option to make the switches reboot in a staggered fashion.
In any case, even if you could stagger the reboots, you still would not have zero downtime, because any hosts connected to one switch would go down when it reboots, and then any hosts on the other switch will also go down when that switch reboots. Does that make sense?
I hope this has been helpful!
Laz