Operation Manual – MSTP
H3C S3100 Series Ethernet Switches Chapter 1 MSTP Configuration
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z A designated port is used to forward packets to a downstream network segment or
switch.
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 A region boundary port is located on the boundary of an MST region and is used to
connect one MST region to another MST region, an STP-enabled region or an
RSTP-enabled region
z An alternate port is a secondary port of a root port or master port and is used for
rapid transition. With the root port or master port being blocked, the alternate port
becomes the new root port or master port.
z A backup port is the secondary port of a designated port and is used for rapid
transition. With the designated port being blocked, the backup port becomes the
new designated port fast and begins to forward data seamlessly. When two ports
of an MSTP-enabled switch are interconnected, the switch blocks one of the two
ports to eliminate the loop that occurs. The blocked port is the backup port.
In
Figure 1-5, switch A, switch B, switch C, and switch D form an MST region. Port 1
and port 2 on switch A 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:
z A port can play different roles in different MSTIs.
z The role a region boundary port plays in an MSTI is consistent with the role it plays
in the CIST. The master port, which is a root port in the CIST while a master port in
the other MSTIs, is an exception.
z For example, in Figure 1-5, port 1 on switch A is a region boundary port. It is a root
port in the CIST while a master port in all the other MSTIs in the region.
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