In the exhibit you also we FastEthernet port is connecting to GigabitEthernet port. The receiving switch will add its local port cost value to the cost in the BPDU. This can be calculated from the 'cost to the root bridge' of each switch because a switch always advertises its cost to the root bridge in its BPDU. SwitchB will choose the interface with lower cost to the root bridge as the root port so we must calculate the cost on interface Gi0/1 & Gi0/2 of SwitchB to the root bridge. Below are the cost values you should memorize: The higher the bandwidth on a link, the lower the value of its cost. In general, 'cost' is calculated based on bandwidth of the link. Now we come to the most difficult part of this question: SwitchB must have a root port so which port will it choose? To answer this question we need to know about STP cost and port cost. Switch C has lowest MAC address so it will become root bridge and 2 of its ports (Fa0/1 & Fa0/2) will be designated ports -> E is incorrect.īecause SwitchC is the root bridge so the 2 ports nearest SwitchC on SwitchA (Fa0/1) and SwitchD (Gi0/2) will be root ports -> B and F are correct. The question says 'no other configuration changes have been made' so we can understand these switches have the same bridge priority.

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