3Com WX3000 operation manual Step Description

Step Description

2The device compares the configuration BPDUs of all the ports and chooses the optimum configuration BPDU.

Principle for configuration BPDU comparison:

z

z

z

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.

zSelection 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.

zSelection of the root port and designated ports

The process of selecting the root port and designated ports is as follows:

Table 1-3Selection of the root port and designated ports

Step Description

1A non-root-bridge device takes the port on which the optimum configuration BPDU 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.

z The root bridge ID is replaced with that of the configuration BPDU of the root port.

2z 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.

zThe designated bridge ID is replaced with the ID of this device.

zThe 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:

zIf the calculated configuration BPDU is superior, this port will serve as the designated

3port, and the configuration BPDU on the port will be replaced with the calculated configuration BPDU, which will be sent out periodically.

zIf 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.

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