H3C Technologies H3C WX6000 Series Access Controllers - Manual (Page 170)

20-12
8) Common root bridge
The common root bridge is the root bridge of the CIST.
In Figure 20-4 , for example, the common root bridge is a device in region A0.
9) Boundary port
A boundary port is a port that connects an MST region to another MST configuration, or to a single
spanning-tree region running STP, or to a single spanning-tree region running RSTP.
During MSTP calculation, a boundary port assumes the same role on the CIST and on MST instances.
Namely, if a boundary port is the master port on the CIST, it is also the master port on all MST instances
within this region. In
Figure 20-4 , for example, 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.
Currently, the device is not capable of recognizing boundary ports. When the device interworks with a
third party’s device that supports boundary port recognition, the third party’s device may malfunction in
recognizing a boundary port.
10) Roles of ports
In the MSTP calculation process, port roles include root port, designated port, master port, alternate
port, backup port, and so on.
z
Root port: a port responsible for forwarding data to the root bridge.
z
Designated port: a port responsible for forwarding data to the downstream network segment or
device.
z
A master port connects an MST region to the common root. The path from the master port to the
common root is the shortest path between the MST region and the common root. In the CST, the
master port is the root port of the region, which is considered as a node. The master port is a
special boundary port. It is a root port in the IST/CIST while a master port in the other MSTIs.
z
Alternate port: The standby port for the root port or master port. When the root port or master port is
blocked, the alternate port becomes the new root port or master port.
z
Backup port: The backup port of designated ports. When a designated port is blocked, the backup
port becomes a new designated port and starts forwarding data without delay. When a loop occurs
while two ports of the same MSTP device are interconnected, the device will block either of the two
ports, and the backup port is that port to be blocked.
A port can assume different roles in different MST instances.