CCNA Practice Exam Test Sample – Question 53

Refer to the topology shown in the exhibit.
q53-q
Which three ports will be STP designated ports if all the links are operating at the same bandwidth? (Choose three.)

A. Switch A – Fa0/0
B. Switch A – Fa0/1
C. Switch B – Fa0/0
D. Switch B – Fa0/1
E. Switch C – Fa0/0
F. Switch C – Fa0/1

Correct Answers: B,C,D
Section: LAN Switching Technologies

Explanation:
This question is to check the spanning tree election problem.
1. First, select the root bridge, which can be accomplished by comparing the bridge ID, the smallest will be selected. Bridge-id= bridge priority + MAC address. The three switches in the figure all have the default priority, so we should compare the MAC address, it is easy to find that SwitchB is the root bridge.
2. Select the root port on the non-root bridge, which can be completed through comparing root path cost. The smallest will be selected as the root port.
3. Next, select the Designated Port. First, compare the path cost, if the costs happen to be the same, then compare the BID, still the smallest will be selected. Each link has a DP. Based on the exhibit above, we can find DP on each link. The DP on the link between SwitchA and SwitchC is SwitchA’Fa0/1, because it has the smallest MAC address.