Two Catalyst 3750G stacked together and running properly.
If I want to break the stack between two switches online ( when the two switches are still running together ). Is it possible or caused any network down ? During the stack breaking, are two switches running properly ? after the stack cables are separated, are they running individually without any network down ( two switches are connected individual by a trunking cable to other switch ) ? Before the breaking, do I have to stop the stack module ? how to stop it ?
If you break the stack, both switches will continue working. Since both switches both have a connection to another switch, disconnecting them without rebooting one of them is not good idea. I'm not sure what will happen, but I'm sure at a minimum it will cause some type of down time on everything connected to it via layer 2 because spanning-tree will need to reconverge.
It is NEVER a good idea to make any kind of change during the production day, especially if you don't know what the repercussions are.
You can't stop stacking on a 3750, and the obvious question is why you want to do this in the first place.
A couple of things to consider (plus this is all theory, as I have not tried it myself).
Splitting a stack results in TWO switches that are configured for operation as a stack and they therefore RETAIN their Configurations and port enumeration, but just loose their (stack based) connectivity. BOTH switches should do this with minimal disruption to service.
This means that (as an example) switch 1 will have ports Gi1/0/xx and switch 2 will have ports Gi2/0/xx. The Slave 3750 will also become a Master. The MAC Table should be the same on BOTH switches as at the time of the break, MINUS the other switch table entries. Thus the switches can continue operation.
The only part I am not sure about is for a restart - "What would happen if the SLAVE switch is configured to boot its IOS from the first switch"? This may cause problems...
Thus, if I was going to break a stack I would ensure EACH 3750 had its own IOS stored local and its BOOT ROM statement was set correctly for standalone operation, and not rely on another switch for its BOOT image.
Restarting EITHER switch should result in the EXACT SAME configuration (IE Switch 1 should have ports Gi1/0/xx and Switch 2 should have Gi2/0/xx, as the STACK MEMBERS already have enumerated the ports for operation in a stacked environment.
Plus - I would NEVER do this purposefully in a LIVE environment. Stacks are great, but tread carefully when playing with them.