The switch can be partitioned into multiple virtual bridges. Each virtual bridge can run an independent Spanning Tree instance. Each Spanning Tree instance is called a STPD. Each STPD has its own root bridge and active path. After an STPD is created, one or more VLANs can be assigned to it.
A physical port can belong to multiple STPDs. In addition, a VLAN can span multiple STPDs.
The key points to remember when configuring VLANs and STP are:
Each VLAN forms an independent broadcast domain.
STP blocks paths to create a loop-free environment.
Within any given STPD, all VLANs belonging to it use the same spanning tree.