An MSTP network consists of either individual MSTP regions connected to the rest of the network with 802.1D and 802.1w bridges or as individual MSTP regions connected to each other.
An MSTP region defines the logical boundary of the network. With MSTP, you can divide a large network into smaller areas similar to an OSPF area or a BGP Autonomous System, which contain a group of switches under a single administration. Each MSTP region has a unique identifier and is bound together by one CIST that spans the entire network. A bridge participates in only one MSTP region at a time.
An MSTP region can hide its internal STPDs and present itself as a virtual 802.1w bridge to other interconnected regions or 802.1w bridges because the port roles are encoded in 802.1w and MSTP BPDUs.
By default, the switch uses the MAC address of the switch to generate an MSTP region. Since each MAC address is unique, every switch is in its own region by default. For multiple switches to be part of an MSTP region, you must configure each switch in the region with the same MSTP region identifiers. See Configuring MSTP Region Identifiers for information.
In Sample MSTP Topology with Two MSTP Regions, all bridges inside MSTP regions 1 and 2 are MSTP bridges; bridges outside of the regions are either 802.1D or 802.1w bridges.