3Com WX3000 Series Operation Manual page 173

4) Path cost
Path cost is a value used for measuring link capacity. By comparing the path costs of different links, STP
selects the most robust links and blocks the other links to prune the network into a tree.
How STP works
STP identifies the network topology by transmitting configuration BPDUs between network devices.
Configuration BPDUs contain sufficient information for network devices to complete the spanning tree
calculation. Important fields in a configuration BPDU include:
Root bridge ID, consisting of root bridge priority and MAC address.
Root path cost, the cost of the shortest path to the root bridge.
Designated bridge ID, designated bridge priority plus MAC address.
Designated port ID, designated port priority plus port name.
Message age: lifetime for the configuration BPDUs to be propagated within the network.
Max age, lifetime for the configuration BPDUs to be kept in the device.
Hello time, configuration BPDU interval.
Forward delay, forward delay of the port.
For the convenience of description, the description and examples below involve only four parts of a
configuration BPDU:
Root bridge ID (in the form of device priority)
Root path cost
Designated bridge ID (in the form of device priority)
Designated port ID (in the form of port name)
1) Detailed calculation process of the STP algorithm
Initial state
Upon initialization of a device, each device generates a BPDU with itself as the root bridge, in which the
root path cost is 0, designated bridge ID is the device ID, and the designated port is the local port.
Selection of the optimum configuration BPDU
Each device sends out its configuration BPDU and receives configuration BPDUs from other devices.
The process of selecting the optimum configuration BPDU is as follows:
Table 1-2 Selection of the optimum configuration BPDU
Step
Upon receiving a configuration BPDU on a port, the device performs the following
processing:
If the received configuration BPDU has a lower priority than that of the configuration
BPDU generated by the port, the device will discard the received configuration BPDU
1
without doing any processing on the configuration BPDU of this port.
If the received configuration BPDU has a higher priority than that of the configuration
BPDU generated by the port, the device will replace the content of the configuration
BPDU generated by the port with the content of the received configuration BPDU.
Description
1-3