STP Priority : Why is it 32768 by default? Why is it increased or decreased by 4096?

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[@lakshmijampani1067]

Good information

[@Travel2Heal]

Very good information Laksh.. interview question it is!

[@178870]

Good One Laksh !!

[@hksvaru3024]

In the context of the Spanning Tree Protocol (STP), the default value of 32768 and the increment/decrement by 4096 are related to the Path Cost parameter, which is used to determine the best path for data forwarding in a switched network.
Default Path Cost Value of 32768:
The default Path Cost value of 32768 is assigned to ports when no specific Path Cost value is configured manually.
This value is chosen to provide a reasonable initial Path Cost for Ethernet links operating at lower speeds (e.g., 10 Mbps or 100 Mbps).
Increment/Decrement by 4096:
The Path Cost is an inverse metric, which means that lower values are preferred for faster links, and higher values are assigned to slower links.
The Path Cost is calculated based on the link speed, and the values are incremented or decremented by 4096 to create a suitable range for different link speeds.
The increment/decrement by 4096 is based on the Port Cost calculation method defined in the IEEE 802.1D standard for STP.
The increment/decrement by 4096 ensures that there is a sufficient range of Path Cost values to represent different link speeds accurately. This range allows faster links to have lower Path Cost values compared to slower links, enabling STP to select the fastest available path for data forwarding.
Here's an example of how the Path Cost values are assigned based on the link speed:
For a 10 Mbps link, the Path Cost is 32768 (default value)
For a 100 Mbps link, the Path Cost is 32768 - 4096 = 28672
For a 1 Gbps link, the Path Cost is 28672 - 4096 = 24576
For a 10 Gbps link, the Path Cost is 24576 - 4096 = 20480
For a 40 Gbps link, the Path Cost is 20480 - 4096 = 16384
And so on, with lower Path Cost values assigned to faster links.
By using this Path Cost calculation method, STP can select the fastest available path for data forwarding, which helps optimize network performance and avoid potential loops in the switched network topology.

[@laksh5792]

The question again is, why is it 32768 and not any other value? Why is it decreased or increased by 4096? The video answers this question.