A multiple spanning tree region (MST region) consists of multiple devices in a switched network and the
network segments among them. All these devices have the following characteristics:
Same region name
Same VLAN-to-instance mapping configuration
Same MSTP revision level configuration
Physically linked with one another
Multiple MST regions can exist in a switched network. You can assign multiple devices to the same MST
region 4, and all devices in each MST region have the same MST region configuration.
MSTP can generate multiple independent spanning trees in an MST region, and each spanning tree is
mapped to the specific VLANs. Each spanning tree is referred to as a "multiple spanning tree instance
15, for example, MST region 3 comprises three MSTIs, MSTI 1, MSTI 2, and MSTI 0.
VLAN-to-instance mapping table
As an attribute of an MST region, the VLAN-to-instance mapping table describes the mapping
relationships between VLANs and MSTIs.
15, for example, the VLAN-to-instance mapping table of MST region 3 is: VLAN 1 to MSTI 1,
VLAN 2 and VLAN 3 to MSTI 2, and other VLANs to MSTI 0. MSTP achieves load balancing by means
of the VLAN-to-instance mapping table.
The common spanning tree (CST) is a single spanning tree that connects all MST regions in a switched
network. If you regard each MST region as a device, the CST is a spanning tree calculated by these
devices through STP or RSTP.
For example, the blue lines in
An internal spanning tree (IST) is a spanning tree that runs in an MST region. It is a special MSTI, and is
also called "MSTI 0." All VLANs are mapped to MSTI 0 by default. As shown in
IST in MST region 3.
Jointly constituted by ISTs and the CST, the common and internal spanning tree (CIST) is a single
spanning tree that connects all devices in a switched network. ISTs in all MST regions and the CST jointly
constitute the CIST of the entire network. In
inter-region CST constitute the CIST of the entire network.
The root bridge of the IST or an MSTI within an MST region is the regional root of the IST or MSTI. Based
on the topology, different spanning trees in an MST region might have different regional roots.
For example, in MST region 3 in
MSTI 2 is Device C, and the regional root of MSTI 0 (also known as the IST) is Device A.
14, the switched network comprises four MST regions, MST region 1 through MST
15, the regional root of MSTI 1 is Device B, the regional root of
represent the CST.
14, for example, the ISTs in all MST regions plus the
14, MSTI 0 is the