H3C Technologies H3C WX6000 Series Access Controllers - Manual (Page 162)

20-4
For the convenience of description, the description and examples below involve only four parts of a
configuration BPDU:
z
Root bridge ID (in the form of device priority)
z
Root path cost
z
Designated bridge ID (in the form of device priority)
z
Designated port ID (in the form of port name)
1) Specific calculation process of the STP algorithm
z
Initial state
Upon initialization of a device, each port 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.
z
Selection of the optimum configuration BPDU
Each device sends out its configuration BPDUs and receives configuration BPDUs from other devices.
The process of selecting the optimum configuration BPDU is as follows:
Table 20-2 Selection of the optimum configuration BPDU
Step Description
1
Upon receiving a configuration BPDU on a port, the device performs the following processing:
z 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.
z 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.
2
The device compares the configuration BPDUs of all the ports and chooses the optimum
configuration BPDU.
Principle for configuration BPDU comparison:
z
The configuration BPDU that has the lowest root bridge ID has the highest priority.
z
If all the 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.
z
If all configuration BPDUs have the same root path cost, they will be compared for their designated
bridge IDs, then their designated port IDs, and then the IDs of the ports on which they are received.
The smaller the ID, the higher message priority.
z
Selection of the root bridge
At network initialization, each STP-compliant device on the network assumes itself to be the root bridge,
with the root bridge ID being its own device ID. By exchanging configuration BPDUs, the devices