MSTI 1, VLAN 2 to MSTI 2, and the rest to CIST. MSTP achieves load balancing by means of the
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 a, the CIST has a section in each MST region, and this section is the IST in the respective MST region.
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 device, the CST is a spanning tree calculated by these devices through STP or RSTP.
The red lines in a represent the CST.
CIST
The common and internal spanning tree (CIST) is a single spanning tree that connects all devices in a switched network. It consists of the ISTs in all MST regions and the CST.
In a, the ISTs (MSTI 0) in all MST regions plus the
MSTI
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).
In a, for example, multiple MSTIs can exist in each MST region, each MSTI corresponding to the specified VLANs.
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.
In region D0 in a, the regional root of MSTI 1 is device B, while that of MSTI 2 is device C.
Common root bridge
The common root bridge is the root bridge of the CIST.
In a, the common root bridge is a device in region A0.
Boundary port
A boundary port is a port that connects an MST region to another MST region, or to a single
During MSTP calculation, the role of a boundary port in an MSTI must be consistent with its role in the CIST. But this is not true with master ports. A master port on MSTIs is a root port on the CIST. For example, in a, 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.
Port roles
MSTP calculation involves the following port roles:
∙Root port: Forwards data for a
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