Here’s a clean reply you can post in that thread:
Short answer: technically yes, but don’t rely on it in production without a maintenance window.
For Catalyst 9200, the StackWise modules themselves are generally considered hot-swappable hardware, meaning you can physically insert/remove them while the switch is powered on without damaging anything. (Holcon Systems LTD)
However, Cisco doesn’t clearly position stack modules the same way as things like power supplies or uplink modules (which are explicitly documented as hot-swappable), so behavior can vary depending on software and timing. (Cisco)
In practice:
You can usually insert the stack module with the switch running
But forming or modifying the stack (adding switches, cables, etc.) can still trigger stack election, reloads, or instability
Even if it “works live,” you don’t want to risk unexpected disruption on a production switch
That’s why most people (and TAC guidance) still recommend doing this during a maintenance window.
So the safe approach is: power down, install the module, then bring it back up.
The “it works hot” approach is possible, but not something I’d trust unless downtime doesn’t matter.
