3Com E4500-24 Cli Configuration Manual page 201

Table 1-2 Selection of the optimum configuration BPDU
Step
1
2
Principles 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 fields are compared
sequentially: designated bridge IDs, designated port IDs, and then the IDs of the ports on which the
configuration BPDUs are received. The smaller these values, the higher priority for the
configuration BPDU.
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 bridge ID. By exchanging configuration BPDUs, the devices
compare one another's configuration BPDU priority. The device with the highest configuration BPDU
priority is elected as the root bridge.
Selection of the root port and designated ports
The process of selecting the root port and designated ports is as follows:
Table 1-3 Selection of the root port and designated ports
Step
A non-root-bridge device takes the port on which the optimum configuration BPDU
1
was received as the root port.
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.
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
Description
1-5