3Com WX3000 Series Operation Manual page 175

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 1-2 Network diagram for STP algorithm
Device A
With priority 0
AP 1
5
BP 1
BP 2
Device B
With priority 1
Initial state of each device
The following table shows the initial state of each device.
Table 1-4 Initial state of each device
Device
Device A
Device B
Device C
Comparison process and result on each device
The following table shows the comparison process and result on each device.
AP 2
10
4
CP 1
CP 2
Device C
With priority 2
Port name
AP1
AP2
BP1
BP2
CP1
CP2
1-5
BPDU of port
{0, 0, 0, AP1}
{0, 0, 0, AP2}
{1, 0, 1, BP1}
{1, 0, 1, BP2}
{2, 0, 2, CP1}
{2, 0, 2, CP2}