3Com 5500-EI PWR Install Manual page 216

Step
Based on the configuration BPDU and the path cost of the root port, the device
calculates a designated port configuration BPDU for each of the rest ports.
The root bridge ID is replaced with that of the configuration BPDU of the root
port.
2
The root path cost is replaced with that of the configuration BPDU of the root
port plus the path cost corresponding to the root port.
The designated bridge ID is replaced with the ID of this device.
The designated port ID is replaced with the ID of this port.
The device compares the calculated configuration BPDU with the configuration
BPDU on the port whose role is to be determined, and acts as follows based on the
comparison result:
If the calculated configuration BPDU is superior, this port will serve as the
designated port, and the configuration BPDU on the port will be replaced with
3
the calculated configuration BPDU, which will be sent out periodically.
If the configuration BPDU on the port is superior, the device stops updating the
configuration BPDUs of the port and blocks the port, so that the port only
receives configuration BPDUs, but does not forward data or send configuration
BPDUs.
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. At this stage, "STP
convergence" is complete.
2) Example of how the STP algorithm works
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
Initial state of each device
Description
1-6