3Com WX3000 Series Operation Manual page 174

Step
The device compares the configuration BPDUs of all the ports and chooses the optimum
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configuration BPDU.
Principle for configuration BPDU comparison:
The configuration BPDU that has the lowest root bridge ID has the highest priority.
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.
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 device with a higher priority is elected as the
root bridge.
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 root bridge ID. The device with the smallest root bridge ID 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 was
1
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.
The device compares the calculated configuration BPDU with the configuration BPDU
on the port whose role is to be determined, and acts as follows based on the comparison
result:
If the calculated configuration BPDU is superior, this port will serve as the designated
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port, and the configuration BPDU on the port will be replaced with the calculated
configuration BPDU, which will be sent out periodically.
If the configuration BPDU on the port is superior, the device stops updating the
configuration BPDUs of the port and blocks the port, so that the port only receives
configuration BPDUs, but does not forward data or send configuration BPDUs.
Description
Description
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