HP A3100-8 v2 SI Configuration Manual page 64

Common root bridge
The common root bridge is the root bridge of the CIST.
In Figure 14, for example, the common root bridge is a device in MST region 1.
Roles of ports
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 16 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
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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
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blocked, the alternate port takes over.
Backup port: The backup port of a designated port. When the designated port fails, the backup
•
port takes over. When a loop occurs because of the interconnection of two ports of the same MSTP
device, the device blocks either of the two ports, and the blocked port is the backup port.
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
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master port 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 CIST. But
that is not true with master ports. A master port on MSTIs is a root port on the CIST.
Figure 16, an MST region comprises Device
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