Restrictions for Spanning Tree Protocol
An attempt to configure a device as the root device fails if the value necessary to be the root device is less than 1.
If your network consists of devices that support and do not support the extended system ID, it is unlikely that the device with the extended system ID support will become the root device. The extended system ID increases the device priority value every time the VLAN number is greater than the priority of the connected devices running older software.
The root device for each spanning tree instance should be a backbone or distribution device. Do not configure an access device as the spanning tree primary root.