H3C Technologies H3C WX5500E Series Access Controllers - Manual (Page 75)

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IST
An 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 17 , MSTI 0 is the IST in MST region 3.
CIST
The 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 17 , 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 might have different regional roots.
In MST region 3 in
Figure 18 , the regional root of MSTI 1 is Device B, the regional root of MSTI 2 is Device
C, and the regional root of MSTI 0 (also known as the IST) is Device A.
Common root bridge
The common root bridge is the root bridge of the CIST.
In
Figure 17 , the common root bridge is a device in MST region 1.
Port roles
A port can play different roles in different MSTIs. As shown in Figure 19 , an MST region comprises Device
A, Device B, Device C, and Device D. Port A1 and Port A2 of Device A connect to the common root
bridge. Port B2 and Port B3 of Device B form a loop. Port C3 and Port C4 of Device C connect to other
MST regions. Port D3 of Device D directly connects to a host.
Figure 19 Port roles