As we learn in early topics, a value calledSpanning Tree Path Cost Value is used for the purpose of identifying the Root Port. This identification of the Root Port is for transmitting the data. For this, we have to identify the root port in every switch other than the root switch. If there is more than one ports are present in the switch, it is necessary to find the best port from the all available ports.

This is done by calculating the value called Path Coast Value. The best path from the port to the destination device is selected on the basis of the value. The path which has the minimum cost will be selected and known as the Root Port.

The bandwidth and the Spanning Tree Path Cost Value are inversely proportional. That is, if a path has having high bandwidth will have the minimum coast value and it is more preferable than any other. Like, the path having the lowest bandwidth will have a large cost value and therefore, it is less preferable.

The following are the most value for the path with their corresponding bandwidth.

Link Speed: 10 Gbps  – Cost Value: 2

Link Speed: 1 Gbps  – Cost Value: 4

Link Speed: 100 Mbps  – Cost Value: 19

Link Speed: 10 Mbps  – Cost Value: 100

We can explain the concept of cost value by the following  diagram.

Consider that switch 4 wants to transmit the packet to the Root Switch, switch 1, then there two paths occur. Either the packet can be transmitted through the switch 2 or through the switch 3. If we opted for the first path, the Path Cost Value of the Spanning Tree will be 19 + 4 = 23. If we choose the second path for transmission, then the Path Cost Value of the Spanning Tree will be 4 + 4 = 8. Therefore, we choose the second path. Because it has the lowest cost value.

Then the packet from switch 4 to switch 1 will be transmitted through the switch 3. In this example, the root port in the switch 4 will be the port which is connected to the switch 3.

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Finally, we can define the Spanning three path cost value as it is the sum of ports cost value from the switch which transmits the data to the Root Switch. When a BPDU reaches a switch, its path cost value will be added to the cost of the incoming port.