As an attribute of an MST region, the
In Figure 176, the
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 Figure 176 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 Figure 176, the CIST has a section in each MST region, and this section is the IST in the respective MST region.
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 Figure 176, the ISTs in all MST regions plus the
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 might have different regional roots.
As shown in Figure 176, the regional root of MSTI 1 in region D0 is device B, and that of MSTI 2 is device C.
Common root bridge
The common root bridge is the root bridge of the CIST.
In Figure 176, for example, the common root bridge is a device in region A0.
Port roles
A port can play different roles in different MSTIs. As shown in Figure 177, an MST region has device A, device B, device C, and device D. Port 1 and port 2 of device A are connected to the common root bridge, port 5 and port 6 of device C form a loop, and port 3 and port 4 of device D are connected downstream to the other MST regions.
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