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. In
table of region A0 is as follows: VLAN 1 is mapped to MSTI 1, VLAN 2 to MSTI 2, and the rest to CIST.
MSTP achieves load balancing by means of the VLAN-to-instance mapping table.
An internal spanning tree (IST) is a spanning tree that runs in an MST region.
ISTs in all MST regions and the common spanning tree (CST) jointly constitute the common and internal
spanning tree (CIST) of the entire network. An IST is the section of the CIST in an MST region, as
The 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 red lines in
Jointly constituted by ISTs and the CST, the CIST is a single spanning tree that connects all devices in a
1-4, for example, the ISTs in all MST regions plus the inter-region CST constitute the CIST of
the entire network.
Multiple spanning trees can be generated in an MST region through MSTP, one spanning tree being
independent of another. Each spanning tree is referred to as a multiple spanning tree instance (MSTI).
1-4, for example, multiple spanning trees can exist in each MST region, each spanning tree
corresponding to the specific VLAN(s). These spanning trees are called MSTIs.
Regional root bridge
The root bridge of the IST or an MSTI within an MST region is the regional root bridge of the IST or the
MSTI. Based on the topology, different spanning trees in an MST region may have different regional
For example, in region D0 in
Common root bridge
The common root bridge is the root bridge of the CIST.
1-4, for example, the common root bridge is a device in region A0.
A boundary port is a port that connects an MST region to another MST region, or to a single
spanning-tree region running STP, or to a single spanning-tree region running RSTP. In
example, if a device in region A0 is interconnected with the first port of a device in region D0 and the
common root bridge of the entire switched network is located in region A0, the first port of that device in
region D0 is the boundary port of region D0.
represent the CST.
1-4, the regional root of MSTI 1 is device B, while that of MSTI 2 is
1-4, for example, the VLAN-to-instance mapping