3com WX3000 User Manual

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A region edge port is located on the edge 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  
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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. 
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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 device are interconnected, the 
device blocks one of the two ports to eliminate the loop that occurs. The blocked port is the backup 
port.  
In 
, device A, device B, device C, and device D form an MST region. Port 1 and port 2 on 
device A connect upstream to the common root. Port 5 and port 6 on device C form a loop. Port 3 and 
port 4 on device D connect downstream to other MST regions. This figure shows the roles these ports 
play. 
 
 
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A port can play different roles in different MSTIs. 
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The role a region edge port plays is consistent with the role it plays in the CIST. For example, port 
1 on device A in 
 is a region edge port, and it is a master port in the CIST. So it is a master 
port in all MSTIs in the region. 
 
Figure 1-5 
Port roles 
Edge ports
Connected to the 
common port
Port 1
Port 2
Port 3
Port 4
Port 5
Port 6
MST 
region
Designated 
port
Master 
port
Backup port
Alternate port
A
B
D
C
 
 
Port state 
In MSTP, a port can be in one of the following three states: