3com S7906E Manuel De Montage

 

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Topology change notification (TCN) BPDUs, used for notifying the concerned devices of network 
topology changes, if any. 

A tree network must have a root; hence the concept of root bridge was introduced in STP. 

There is one and only one root bridge in the entire network, and the root bridge can change along with 
changes of the network topology. Therefore, the root bridge is not fixed. 

Upon initialization of a network, each device generates and sends out BPDUs periodically with itself as 
the root bridge; after network convergence, only the root bridge generates and sends out configuration 
BPDUs at a certain interval, and the other devices just forward the BPDUs.  

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 this 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. 

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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. 

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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.