H3C S12500X-AF Series, Configuration Manual

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The regional root of MSTI 1 is Device B.
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The regional root of MSTI 2 is Device C.
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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 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 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
MSTP calculation involves the following port roles:
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Root port
—Forwards data for a non-root bridge to the root bridge. The root bridge does not
have any root port.
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Designated port
—Forwards data to the downstream network segment or device.
•
Alternate port
—Acts as the backup port for a root port or master port. When the root port or
master port is blocked, the alternate port takes over.
•
Backup port
—Acts as the backup port of a designated port. When the designated port is
invalid, the backup port becomes the new designated port. A loop occurs when two ports of the
same spanning tree device are connected, so the device blocks one of the ports. The blocked
port acts as the backup.
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Edge port
—Directly connects to a user host rather than a network device or network segment.
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Master port
—Acts as a port on the shortest path from the local MST region to the common root
bridge. The master port is not always located on the regional root. It is a root port on the IST or
CIST and still a master port on the other MSTIs.
•
Boundary port
—Connects an MST region to another MST region or to an STP/RSTP-running
device. In MSTP calculation, a boundary port's role on an MSTI is consistent with its role on the
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