H3C Technologies H3C SecPath F1000-E - Manual (Page 92)

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During MSTP calculation, the role of a boundary port in an MSTI must be consistent with its role in the
CIST. But this is not true with master ports. A master port on MSTIs is a root port on the CIST. For example,
in
Figure 42 , if a device in region A0 is connected to the first port of a device in region D0, and the
common root bridge of the entire switched network is located in region A0, the first port of that device
in region D0 is the boundary port of region D0.
Roles of ports
MSTP calculation involves the following port roles:
• Root port—a port responsible for forwarding data to the root bridge.
• Designated port—a port responsible for forwarding data to the downstream network segment or
device.
• Master port—a port on the shortest path from the current region to the common root bridge,
connecting the MST region to the common root bridge. If the region is seen as a node, the master
port is the root port of the region on the CST. The master port is a root port on IST/CIST and still a
master port on the other MSTIs.
• Alternate port—the standby port for the root port and the master port. When the root port or master
port is blocked, the alternate port becomes the new root port or master port.
• Backup port—the backup port of a designated port. When the designated port is blocked, the
backup port becomes a new designated port and starts forwarding data without delay. A loop
occurs when two ports of the same MSTP device are interconnected. The device will block either of
the two ports, and the backup port is the port to be blocked.
A port can play different roles in different MSTIs.
Figure 43 Port roles
In Figure 43 , devices A, B, C, and D constitute an MST region. Port 1 and port 2 of device A are
connected to the common root bridge, port 5 and port 6 of device C form a loop, port 3 and port 4 of
device D are connected downstream to the other MST regions.
Connecting to the
common root bridge
Edge port
Port 1
Master port Alternate port
Designated
port
A
B C
D
Backup port
MST region
Port 2
Port 6
Port 5
Port 4 Port 3