Switches with different Cisco IOS software versions likely have different stack protocol versions. Switches with different major stack protocol version numbers are incompatible and cannot exist in the same switch stack.
Switches with the same major version number but a different minor version number as the stack master are considered partially compatible. When connected to a switch stack, partially compatible switches enter into version mismatch (VM) mode and cannot join the stack. The stack master downloads the software version it uses to any switch in VM mode.
When connected to a switch stack, a partially compatible switch enters version-mismatch (VM) mode with the %STACKMGR-6-SWITCH_ADDED_VM error message. This message indicates that a switch with a different software version is added to the stack.
To check whether any switches are in the VM mode, issue the show switch user EXEC command.
Make sure that all stack members run the same version of Cisco IOS Software. This helps:
To ensure that all stack members are compatible.
To ensure full compatibility in the stack protocol version among stack members. If the software detects any mismatch and tries to upgrade or downgrade the switch in VM mode, and uses the switch stack image or a tar file image from the Flash memory of the switch stack. The software uses the automatic upgrade (auto-upgrade) and the automatic advise (auto-advise) features.
The failure of the auto-upgrade feature requires a manual upgrade or downgrade of the newly added stack member to the exact version that runs in the switch stack master. To do this, refer to the Software download page.