Step

Description

 

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 other ports.

 

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

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• The root path cost is replaced with that of the configuration BPDU of the root port plus

 

the path cost of 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 port role will be determined, and acts depending on the result of the comparison:

•If the calculated configuration BPDU is superior, the device considers this port as the

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designated port, replaces the configuration BPDU on the port with the calculated

 

configuration BPDU, and periodically sends the calculated configuration BPDU.

•If the configuration BPDU on the port is superior, the device blocks this port without updating its configuration BPDU. The blocked port can receive BPDUs, but it cannot send BPDUs or forward any data.

When the network topology is stable, only the root port and designated ports forward user traffic. Other ports are all in the blocked state to receive BPDUs but not to forward BPDUs or user traffic.

Table 54 Selecting the optimum configuration BPDU

StepActions

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Upon receiving a configuration BPDU on a port, the device compares the priority of the received configuration BPDU with that of the configuration BPDU generated by the port. It takes one of the following actions:

•If the former priority is lower, the device discards the received configuration BPDU and keeps the configuration BPDU the port generated.

•If the former priority is higher, the device replaces 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.

The following are the principles of configuration BPDU comparison:

a.The configuration BPDU with the lowest root bridge ID has the highest priority.

b.If all the configuration BPDUs have the same root bridge ID, their root path costs are compared. For example, the root path cost in a configuration BPDU plus the path cost of a receiving port is S. The configuration BPDU with the smallest S value has the highest priority.

c.If all configuration BPDUs have the same root bridge ID and S value, their designated bridge IDs, designated port IDs, and the IDs of the receiving ports are compared in sequence. The configuration BPDU that contains a smaller designated bridge ID, designated port ID, or receiving port ID is selected.

A tree-shape topology forms when the root bridge, root ports, and designated ports are selected.

Example of STP calculation

Figure 174 provides an example showing how the STP algorithm works.

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