In order to stack the DGS-3120 series switches, you should ensure you comply with the following steps:

Stacking hardware A1 and A2:
- You must use the same firmware version on all units.

Stacking mixed hardware Ax and B1:
- Hardware B firmware can only be 500 higher than hardware A firmware (e.g.: hardware A firmware 3.00.022 and hardware B firmware 3.00.522)
- When stacking hardware Ax and B1, it is strongly advisable to have the hard-ware Ax device as master.