HP 6125XLG Configuration Manual page 95

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 23, 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 25 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—Serves 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—Serves 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.
• Edge port—Does not connect to any network device or network segment, but directly connects to a
user host.
Figure 25, an MST region comprises Device
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