H3C S5600 SERIES Operation Manual page 227

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
1
2
Principle for configuration BPDU comparison:
The configuration BPDU that has the lowest root bridge ID has the highest priority.
If all configuration BPDUs have the same root bridge ID, they will be compared for their root path
costs. If the root path cost in a configuration BPDU plus the path cost corresponding to this port is S,
the configuration BPDU with the smallest S value has the highest priority.
If all configuration BPDUs have the same root path cost, the following configuration BPDU priority
is compared sequentially: designated bridge IDs, designated port IDs, and then the IDs of the ports
on which the configuration BPDUs are received. The switch with a higher priority is elected as the
root bridge.
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 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.
The device compares the configuration BPDUs of all the ports and chooses
the optimum configuration BPDU.
Description
1-4