During a migration process to a new network distribution I was stuck with a boring problem. I was unable to stack our new 2960-X with our old 2960-S. When searching the Cisco’s documentation I’ve found a solution that needs to be used before starting the stack…
The 2960-X and the 2960-S do not use the same speed for the stack’s bandwidth, the new ones use 20Gbps end the old ones use 10Gbps. So before stacking your switches you need to downgrade your 2960-X with the command below and reload:
switch stack port-speed 10
Full documentation can be read on the Cisco website.