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MST region
A multiple spanning tree region (MST region) consists of multiple routers in a switched network and the
network segments among them. All these routers have the following characteristics:
•
A spanning tree protocol enabled
•
Same region name
•
Same VLAN-to-instance mapping configuration
•
Same MSTP revision level
•
Physically linked together
Multiple MST regions can exist in a switched network. You can assign multiple routers to the same MST
region. In
, the switched network comprises four MST regions, MST region 1 through MST
region 4, and all routers in each MST region have the same MST region configuration.
MSTI
MSTP can generate multiple spanning trees in an MST region, and each spanning tree is independent of
another and maps to specific VLANs. Each spanning tree is referred to as a multiple spanning tree
instance (MSTI).
In
, 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.
In
, 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.
CST
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 router, the CST is a spanning tree calculated by these
routers through STP or RSTP.
The blue lines in
represent the CST.
IST
An internal spanning tree (IST) is a spanning tree that runs in an MST region. It is also called MSTI 0, a
special MSTI to which all VLANs are mapped by default.
In
, MSTI 0 is the IST in MST region 3.
CIST
The common and internal spanning tree (CIST) is a single spanning tree that connects all routers in a
switched network. It consists of the ISTs in all MST regions and the CST.
In
, the ISTs (MSTI 0) in all MST regions plus the inter-region CST constitute the CIST of the entire
network.
Regional root
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 may have different regional roots.
