97
The blue lines in Figure 30 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 30, 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 devices in
a switched network. It consists of the ISTs in all MST regions and the CST.
In Figure 30, the IST
s (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,
as shown in MST region 3 in Figure 31.
• The re
gional root of MSTI 1 is Device B.
• The regional root of MSTI 2 is Device C.
• 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 30, the comm
on root bridge is a device in MST region 1.
Port roles
A port can play different roles in different MSTIs. As shown in Figure 32, an MST region contains
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 32 Port roles
Device A
(Root bridge)
Port A1 Port A2
Root port
Designated port
Normal link
Blocked link
Alternate port
Backup port
Master port
Boundary port
Device C
Device B Device D
Port A3 Port A4
Port B1
Port B2 Port B3
Port C1
Port C2
Port C3 Port C4
Port D1
Port D2
MST region
To the common root
To other MST regions
Edge port
Port D3
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