1-5Table 1-2 Selection of the optimum configuration BPDUStep Description1Upon receiving a configuration BPDU on a port, the device performs thefollowing processing:z If the received configuration BPDU has a lower priority than that of theconfiguration BPDU generated by the port, the device will discard thereceived configuration BPDU without doing any processing on theconfiguration BPDU of this port.z If the received configuration BPDU has a higher priority than that of theconfiguration BPDU generated by the port, the device will replace thecontent of the configuration BPDU generated by the port with the content ofthe received configuration BPDU.2 The device compares the configuration BPDUs of all the ports and choosesthe optimum configuration BPDU.Principles for configuration BPDU comparison:z The configuration BPDU that has the lowest root bridge ID has the highest priority.z If all configuration BPDUs have the same root bridge ID, they will be compared for their root pathcosts. 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, the following fields are comparedsequentially: designated bridge IDs, designated port IDs, and then the IDs of the ports on which theconfiguration BPDUs are received. The smaller these values, the higher priority for theconfiguration BPDU.z Selection of the root bridgeAt network initialization, each STP-compliant device on the network assumes itself to be the root bridge,with the root bridge ID being its own bridge ID. By exchanging configuration BPDUs, the devicescompare one another’s configuration BPDU priority. The device with the highest configuration BPDUpriority is elected as the root bridge.z Selection of the root port and designated portsThe process of selecting the root port and designated ports is as follows:Table 1-3 Selection of the root port and designated portsStep Description1 A non-root-bridge device takes the port on which the optimum configuration BPDUwas received as the root port.2Based on the configuration BPDU and the path cost of the root port, the devicecalculates a designated port configuration BPDU for each of the rest ports.z The root bridge ID is replaced with that of the configuration BPDU of the root port.z The root path cost is replaced with that of the configuration BPDU of the root portplus the path cost corresponding to the root port.z The designated bridge ID is replaced with the ID of this device.z The designated port ID is replaced with the ID of this port.
