HP 6125G Configuration Manual page 69

For example, in MST region 3 in
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 15, for example, 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
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 17 Port roles
MSTP calculation involves the following port roles:
Root port—Forwards data for a non-root bridge to the root bridge. The root bridge does not have
•
any root port.
• Designated port—Forwards data to the downstream network segment or device.
Alternate port—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—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 interconnected, so the device blocks one of the ports. The blocked port
acts as the backup.
Edge port—An edge port does not connect to any network device or network segment, but directly
•
connects to a user host.
Master port—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.
Figure 16, the regional root of MSTI 1 is Device B, the regional root of
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Figure 17, an MST region comprises Device