Question 1hardmultiple choice
Full question →hardmultiple choiceObjective-mapped
An engineer lowers the spanning-tree path cost on one uplink of a nonroot switch. What is the expected result if all else stays equal?
Answer choices
Why each option matters
Good practice is not just finding the correct option. The wrong answers often show the exact trap the exam wants you to fall into.
A
Distractor review
The switch becomes the root bridge immediately.
Path cost on one port does not by itself make the switch the root bridge.
B
Best answer
That uplink becomes more likely to be the root port.
Correct. Lower root-path cost is preferred.
C
Distractor review
All designated ports on downstream switches go to blocking.
Changing one port cost does not cause that blanket behavior.
D
Distractor review
The port stops sending BPDUs.
The port still participates in STP.
Common exam trap
Common exam trap: answer the scenario, not the keyword
A frequent exam trap is assuming that lowering the path cost on one uplink port causes the switch to become the root bridge immediately. This is incorrect because root bridge election depends solely on the lowest bridge ID, not on path costs. Another common mistake is believing that all designated ports on downstream switches will transition to blocking due to a single port cost change, which does not happen. Also, some candidates mistakenly think the port will stop sending BPDUs after cost adjustment, but STP requires continuous BPDU transmission to maintain topology information. Recognizing these misconceptions prevents selecting incorrect answers.
Technical deep dive
How to think about this question
Spanning Tree Protocol (STP) is a Layer 2 protocol designed to prevent switching loops in Ethernet networks by creating a loop-free logical topology. STP elects a single root bridge based on the lowest bridge ID, and all other switches determine their best path to this root bridge. Each non-root switch selects one root port, which is the port with the lowest cumulative path cost to the root bridge. The path cost is calculated based on the bandwidth of the links along the path, with lower costs preferred. When an engineer lowers the spanning-tree path cost on one uplink port of a non-root switch, this action reduces the cumulative cost to reach the root bridge via that port. As a result, the switch is more likely to select that uplink as its root port because STP always prefers the lowest-cost path to the root. This change affects only the local switch’s root port selection and does not influence the root bridge election, which depends solely on bridge IDs. Other ports on downstream switches remain unaffected in their designated or blocking states. A common exam trap is to confuse path cost changes with root bridge election or assume that lowering path cost causes all designated ports downstream to block. In reality, path cost adjustments influence only root port selection on the local switch. Additionally, ports continue to send BPDUs to maintain STP topology information regardless of cost changes. Understanding this distinction helps avoid mistakes during the exam and in practical network troubleshooting, ensuring correct interpretation of STP behavior when modifying path costs.
KKey Concepts to Remember
- Spanning Tree Protocol (STP) selects the root port on a non-root switch as the port with the lowest path cost to the root bridge.
- Lowering the spanning-tree path cost on an uplink port makes that port more likely to be chosen as the root port by the switch.
- The root bridge is elected based on the lowest bridge ID, not by adjusting path costs on individual switches.
- Designated ports on downstream switches forward frames unless blocked by STP to prevent loops; changing one port cost does not block all designated ports.
- Ports continue to send Bridge Protocol Data Units (BPDUs) to maintain STP topology information regardless of path cost changes.
- STP path cost is cumulative and calculated based on interface bandwidth, influencing root port selection on non-root switches.
- A non-root switch uses the lowest root path cost to determine its root port, which is the port that provides the best path to the root bridge.
- Changing the path cost on one uplink affects only the local switch’s root port selection and does not immediately change the root bridge election.
TExam Day Tips
- Watch for words such as best, first, most likely and least administrative effort.
- Review why wrong options are wrong, not only why the correct option is correct.
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More questions from this exam
Keep practising from the same exam bank, or move into a focused topic page if this question exposed a weak area.
Question 1
A router learns the same prefix from both OSPF and EIGRP. Which route is installed by default?
Question 2
A router shows this output: R1#show ip ospf neighbor Neighbor ID Pri State Dead Time Address Interface 10.1.1.2 1 FULL/DR 00:00:34 192.168.12.2 GigabitEthernet0/0 10.1.1.3 1 2WAY/DROTHER 00:00:39 192.168.12.3 GigabitEthernet0/0 Which statement is correct?
Question 3
What is the OSPF metric called?
Question 4
A non-root switch has two uplinks toward the root bridge. One path has a lower total STP cost than the other. What role will the lower-cost uplink have?
Question 5
A router interface applies this ACL inbound: 10 deny tcp any any eq 80 20 permit ip any any A user reports that web browsing to a server by IP address fails, but ping works. Which statement best explains the behavior?
Question 6
A router learns route 198.51.100.0/24 from OSPF with AD 110 and also has a static route to the same prefix configured with AD 150. Which route is installed?
FAQ
Questions learners often ask
What does this 200-301 question test?
Spanning Tree Protocol (STP) selects the root port on a non-root switch as the port with the lowest path cost to the root bridge.
What is the correct answer to this question?
The correct answer is: That uplink becomes more likely to be the root port. — STP prefers the lower-cost path to the root bridge, so that uplink becomes more likely to be selected as the root port.
What should I do if I get this 200-301 question wrong?
Then try more questions from the same exam bank and focus on understanding why the wrong options are tempting.
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