When the network topology is stable, only the root port and designated ports forward traffic, while other ports are all in the blocked state – they only receive STP packets but do not forward user traffic.

Once the root bridge, the root port on each non-root bridge and designated ports have been successfully elected, the entire tree-shaped topology has been constructed.

The following is an example of how the STP algorithm works. The specific network diagram is shown in Figure 1-2. The priority of Device A is 0, the priority of Device B is 1, the priority of Device C is 2, and the path costs of these links are 5, 10 and 4 respectively.

Figure 1-2Network diagram for STP algorithm

 

Device A

 

With priority 0

AP 1

AP 2

5

 

 

10

 

 

 

 

 

BP 1

 

 

 

 

 

BP 2

4

 

 

 

 

CP 2

CP 1

Device B

 

 

 

 

 

 

 

 

 

With priority1

 

 

 

 

 

 

 

 

Device C

 

 

 

 

With priority 2

zInitial state of each device

The following table shows the initial state of each device.

Table 1-4Initial state of each device

 

Device

Port name

BPDU of port

 

Device A

AP1

{0, 0, 0, AP1}

 

 

 

AP2

{0, 0, 0, AP2}

 

 

 

 

 

 

 

Device B

BP1

{1, 0, 1, BP1}

 

 

 

 

BP2

{1, 0, 1, BP2}

 

 

 

 

 

 

 

Device C

CP1

{2, 0, 2, CP1}

 

 

 

 

CP2

{2, 0, 2, CP2}

 

 

 

 

 

 

zComparison process and result on each device

The following table shows the comparison process and result on each device.

1-5

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3Com WX3000 operation manual Device Port name Bpdu of port