3Com WX3000 operation manual How STP works, Step Description

4)Path cost

Path cost is a value used for measuring link capacity. By comparing the path costs of different links, STP selects the most robust links and blocks the other links to prune the network into a tree.

How STP works

STP identifies the network topology by transmitting configuration BPDUs between network devices. Configuration BPDUs contain sufficient information for network devices to complete the spanning tree calculation. Important fields in a configuration BPDU include:

z

z

z

z

z

z

z

z

Root bridge ID, consisting of root bridge priority and MAC address. Root path cost, the cost of the shortest path to the root bridge. Designated bridge ID, designated bridge priority plus MAC address. Designated port ID, designated port priority plus port name.

Message age: lifetime for the configuration BPDUs to be propagated within the network. Max age, lifetime for the configuration BPDUs to be kept in the device.

Hello time, configuration BPDU interval. Forward delay, forward delay of the port.

For the convenience of description, the description and examples below involve only four parts of a configuration BPDU:

z

z

z

z

Root bridge ID (in the form of device priority) Root path cost

Designated bridge ID (in the form of device priority) Designated port ID (in the form of port name)

1)Detailed calculation process of the STP algorithm z Initial state

Upon initialization of a device, each device 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.

zSelection of the optimum configuration BPDU

Each device sends out its configuration BPDU and receives configuration BPDUs from other devices.

The process of selecting the optimum configuration BPDU is as follows:

Table 1-2Selection of the optimum configuration BPDU

Step Description

Upon receiving a configuration BPDU on a port, the device performs the following processing:

zIf the received configuration BPDU has a lower priority than that of the configuration

1BPDU generated by the port, the device will discard the received configuration BPDU without doing any processing on the configuration BPDU of this port.

zIf 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.

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