H3C S7500 Series Operation Manual page 248

Operation Manual – MSTP
H3C S7500 Series Ethernet Switches
VIII. Common root bridge
The common root bridge is the root of the CIST. The common root bridge of the network
shown in
IX. Port roles
In MSTP, the following port roles exist: root port, designated port, master port, region
edge port, alternate port, and backup port.
A root port is used to forward packets to the root.
A designated port is used to forward packets to a downstream network segment or
switch.
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.
A region edge port is located on the edge of an MST region and is used to connect
the MST region to another MST region, an STP-enabled region or an
RSTP-enabled region
An alternate port is a backup port of a master port. It becomes the master port if
the existing master port is blocked.
A loop occurs when two ports of a switch are connected to each other. In this case,
the switch blocks one of the two ports. The blocked port is a backup port.
In Figure
connect upstream to the common root. Port 5 and port 6 on switch C form a loop. Port 3
and port 4 on switch D connect downstream to other MST regions. This figure shows
the roles these ports play.
Note:
A port can play different roles in different MSTIs.
The role a region edge port plays is consistent with the role it plays in the CIST. For
example, port 1 on switch A in
port in the CIST. So it is a master port in all MSTIs in the region.
Figure 1-1 is a switch in region A0.
1-2, switch A, B, C, and D form an MST region. Port 1 and port 2 on switch A
Figure 1-2 is a region edge port, and it is a master
1-4
Chapter 1 MSTP Configuration