I have an exsisting 3750 and will be adding a second via the stack cable. I realize that they need to run the same code. I should power them off, hook them up, then boot them. I also realize that the interfaces will change IE from fa 1/0 to fa 1/0/1 etc. Now with that said, what will happen to the exsisting config that is there? Will it all migrate over except for the interface commands?
Normally, this goes seamlessly.
When the switches are both running the same feature-set the flash will be updated automatically upon boot.
Interface numbering should be no problem either. All you need to do is connect the stacking cable and then power the second switch. Beware that it is done in exactly this order and not the other way round!
The primary switch will then become stack master and keep it's interfacenumbering. The new switch will become 2/0/1 - 24 (or 48)
You can monitor the process via the console, do not interfere with it.
Perhaps you should also check the URL below before you start: