Root path cost?

Question

A little confused here. Im trying to determine the root port for SWITCH B. SWITCH C here is the ROOT SWITCH. I understand that to determine the root port you need to find the lowest cost path to the root switch. So at the path highlighted in green, is the root path cost:

1. 38 (because FA0/2 from SWITCH A + FA0/1 from SWITCH C)

or

2. 23 (FA0/1 from SWITCH A + Gi 0/2 from SWITCH B)

I cannot which part should I look at to determine the cost (especially the connection between SW A and SW B). They are connected but on different speed port, one using Fast Ethernet, the other one Gigabit Ethernet?

Answer 1

Switch B's root port will be G0/1. This is because the cost for each link is not what speed the port is capable of running, but the speed it is actually running. In this scenario, the link between Switch C and Switch D has a cost of 19, because the Gigabit port on Switch D will negotiate down to 100Mbps, not stay at 1Gbps. The link between Switch B and Switch D will have a cost of 4 because it will negotiate to 1Gbps. From Switch B's perspective, G0/2 has a root path cost of 38 while G0/1 will have a root path cost of 23.

Answer 2

I think you need to consider the fact that although both interfaces are capable of different speeds, ultimately the link would only come up if it negotiated at the lowest speed supported by both ends.