H3C Technologies H3C SecPath F1000-E - Manual (Page 205)

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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 4 , if a device in region A0 is interconnected with 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 these port roles: root port, designated port, master port, boundary port,
alternate port, backup port, and so on.
• 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. Therefore, 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 5 Port roles
Connecting to the
common root bridge
Port 1
Port 2
Master port Alternate port
Designated port
Port 3 Port 4
Port 5
A
B C
D
Port 6
Backup port
MST region