When the data pass the Switch in port can represent cost value?

Question

The snapshot is a tutorial of Switch.

You see the bellow snapshot, from the ROOT SW1 node to SW3, there are two path:

the author says the the path1(left side path) cost is 0 + SW3-in-port. the path2(right side path) cost is 0 + SW2-in-port + SW3-in-port.

there I have a doubt, why author do not use the passing Switches node as the cost value? if the data passing port can represent cost, why the Switch out port do not have value?

Answer 1

For SW3, the left root path has a cost of 2,000,000, and the right path has a cost of 2,000,000 + 200,000 = 2,200,000. It chooses the lower cost = left path.

If the link between SW2 and SW3 was 100BASE-TX as well, the cost would decrease to 400,000 and SW3 would choose the right-hand path.