3com 2928 User Guide

 

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On a non-root bridge, the port nearest to the root bridge is called the root port. The root port is 
responsible for communication with the root bridge. Each non-root bridge has one and only one root 
port. The root bridge has no root port.  

The following table describes designated bridges and designated ports.  

Description of designated bridges and designated ports:  

For a device 

A device directly connected with the 
local device and responsible for 
forwarding BPDUs to the local device 

The port through which the 
designated bridge forwards BPDUs 
to the local device 

For a LAN  

The device responsible for forwarding 
BPDUs to this LAN segment 

The port through which the 
designated bridge forwards BPDUs 
to this LAN segment 

 

As shown in 

, AP1 and AP2, BP1 and BP2, and CP1 and CP2 are ports on Device A, Device 

B, and Device C respectively. 

z

 

If Device A forwards BPDUs to Device B through AP1, the designated bridge for Device B is Device 
A, and the designated port of Device B is port AP1 on Device A.  

z

 

Two devices are connected to the LAN: Device B and Device C. If Device B forwards BPDUs to the 
LAN, the designated bridge for the LAN is Device B, and the designated port for the LAN is the port 
BP2 on Device B.  

A schematic diagram of designated bridges and designated ports 

 

 

Path cost is a reference value used for link selection in STP. By calculating path costs, STP selects 
relatively robust links and blocks redundant links, and finally prunes the network into a loop-free tree.