@MK2 it is best to connect the cables with the switches off (pre-stage the new one first so it registers to the dashboard), but we have built quite a few stacks 'on the fly' and it usually works.  The process that we follow (when we can't find a downtime slot) is:   Configure stack in dashboard of old + new switch Connect stacking cables from new to old switch Power on new switch with a front port connection to the existing switch to give it access to the dashboard Wait an hour for firmware upgrades/downgrades, registration and config to complete If the stack port lights are both green and the front port connection now shows spanning tree blocking on the old switch end then the stacking has worked Wait up to another hour for any stacking error messages to go away Unplug front port connection and start using new switch in the stack.   This has worked multiple times with MS225/210 stacks and usually works with MS355 stacks, though occasionally a new switch needs a manual reboot to clear the stacking messages.  I have never had to reboot the existing switch so far...   ... View more

@mkhavari the MS425 does not support breakout cables and nor do all of the traditional Meraki switches.  I am not aware whether the MS390 supports them, but it is easy to find out as the module will be assigned 8 port numbers if it does. ... View more

@cxp_tcsd unfortunately not.  Warm spare configuration always has to be the same hardware model.  ... View more