- A
Excessive collisions and CRC errors on the interface
On the half-duplex side, collisions are normal but become excessive due to the full-duplex side transmitting without listening. CRC errors occur when frames are corrupted by collisions.
- B
Runts and frame errors on the interface
On the full-duplex side, collisions are not detected, so the interface sees fragments of frames (runts) and alignment errors due to frames truncated by collisions on the half-duplex side.
- C
Auto-negotiation failed message in the interface output
Why wrong: The 'show interfaces' output does not display 'auto-negotiation failed' as a direct symptom. It shows the negotiated duplex and speed, but a mismatch can occur even if auto-negotiation succeeds (e.g., when one side is manually configured).
- D
High input rate on the interface
Why wrong: Duplex mismatch typically causes high error rates, not high throughput. The actual data rate may be low due to retransmissions, and the input rate shown is usually not elevated.
- E
Line protocol is down
Why wrong: A duplex mismatch does not bring the line protocol down. The interface status remains up/up (or up/down if there is a physical issue), but errors accumulate. Line protocol down indicates a more fundamental problem like no keepalives or a Layer 2 issue.
Quick Answer
The answer is runts and frame errors on the interface. This occurs because a duplex mismatch forces one side to operate in half-duplex, where collisions are normal, while the full-duplex side never defers or backs off. When the half-duplex side detects a collision and sends a jam signal, the full-duplex side receives that truncated data as a runt, and the corrupted frames register as frame errors in the show interfaces output. On the CCNA 200-301 v2 exam, this question tests your ability to differentiate symptoms by link direction: the half-duplex side shows excessive collisions and CRC errors, while the full-duplex side shows runts and frame errors. A common trap is confusing these symptoms with auto-negotiation failures, but remember that a mismatch can occur even after successful negotiation. For a quick memory tip, think “half hurts with collisions, full gets runts.”
CCNA Network Infrastructure and Connectivity Practice Question
This 200-301 practice question tests your understanding of network infrastructure and connectivity. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
Which TWO symptoms are most likely to appear in the output of 'show interfaces' when a duplex mismatch exists between a switch port and a connected host?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue:
"most likely"Why it matters: Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
Excessive collisions and CRC errors on the interface
A duplex mismatch causes collisions on the half-duplex side, resulting in excessive collisions and CRC errors (option A). On the full-duplex side, the host receives truncated frames from the half-duplex side's collisions, leading to runts and frame errors (option B). Option C is incorrect because 'Auto-negotiation failed' would appear only if negotiation itself failed, not from a mismatch after successful negotiation. Option D is wrong because a duplex mismatch typically reduces throughput and causes errors, not a high input rate. Option E is incorrect because the line protocol remains up; duplex mismatch does not bring the line protocol down.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
Excessive collisions and CRC errors on the interface
Why this is correct
On the half-duplex side, collisions are normal but become excessive due to the full-duplex side transmitting without listening. CRC errors occur when frames are corrupted by collisions.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
- ✓
Runts and frame errors on the interface
Why this is correct
On the full-duplex side, collisions are not detected, so the interface sees fragments of frames (runts) and alignment errors due to frames truncated by collisions on the half-duplex side.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
- ✗
Auto-negotiation failed message in the interface output
Why it's wrong here
The 'show interfaces' output does not display 'auto-negotiation failed' as a direct symptom. It shows the negotiated duplex and speed, but a mismatch can occur even if auto-negotiation succeeds (e.g., when one side is manually configured).
- ✗
High input rate on the interface
Why it's wrong here
Duplex mismatch typically causes high error rates, not high throughput. The actual data rate may be low due to retransmissions, and the input rate shown is usually not elevated.
- ✗
Line protocol is down
Why it's wrong here
A duplex mismatch does not bring the line protocol down. The interface status remains up/up (or up/down if there is a physical issue), but errors accumulate. Line protocol down indicates a more fundamental problem like no keepalives or a Layer 2 issue.
Option-by-option analysis
Why each answer is right or wrong
Understanding why wrong answers are wrong — and when they would be correct — is what separates a 750 score from a 900. The 200-301 exam frequently reuses these exact scenarios with slightly different constraints.
✓Excessive collisions and CRC errors on the interfaceCorrect answer▾
Why this is correct
On the half-duplex side, collisions are normal but become excessive due to the full-duplex side transmitting without listening. CRC errors occur when frames are corrupted by collisions.
✗Auto-negotiation failed message in the interface outputWrong answer — click to see why▾
Why this is wrong here
The 'show interfaces' command does not display an 'auto-negotiation failed' message. It shows the negotiated duplex and speed, but a mismatch can occur even when auto-negotiation succeeds if one side is manually configured. The absence of such a message makes this option incorrect.
Why candidates choose this
Students may think that a duplex mismatch always results from auto-negotiation failure, but mismatches can occur even with successful auto-negotiation when manual configuration overrides it. The 'show interfaces' output does not explicitly indicate negotiation failure.
✗High input rate on the interfaceWrong answer — click to see why▾
Why this is wrong here
Duplex mismatch typically causes high error rates and retransmissions, which reduce actual throughput. The input rate shown in 'show interfaces' is usually not elevated; instead, it may be low due to errors. High input rate is not a typical symptom of duplex mismatch.
Why candidates choose this
Some might assume that errors lead to retransmissions, increasing the input rate. However, in practice, the interface counters show errors, not high throughput, and the input rate is often normal or low.
✗Line protocol is downWrong answer — click to see why▾
Why this is wrong here
A duplex mismatch does not cause the line protocol to go down. The interface remains up/up (or up/down if there is a physical issue), but errors accumulate. Line protocol down indicates a more fundamental Layer 2 problem, such as no keepalives or a misconfigured encapsulation.
Why candidates choose this
Students may confuse duplex mismatch with other issues that cause the line protocol to go down, such as a cable fault or misconfiguration. However, duplex mismatch only affects data integrity, not the line protocol state.
Analysis generated from the official 200-301blueprint and verified against question context. The “when correct” sections are what AI assistants cite when candidates ask “what’s the difference between these options?”
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the distinction that 'runts' and 'frame errors' are symptoms of duplex mismatch on the full-duplex side, while 'excessive collisions' and 'CRC errors' appear on the half-duplex side, and candidates may incorrectly assume both symptoms appear on the same interface.
Trap categories for this question
Command / output trap
The 'show interfaces' output does not display 'auto-negotiation failed' as a direct symptom. It shows the negotiated duplex and speed, but a mismatch can occur even if auto-negotiation succeeds (e.g., when one side is manually configured).
Detailed technical explanation
How to think about this question
In a duplex mismatch, the half-duplex side uses CSMA/CD and will detect collisions when the full-duplex side transmits without listening. The half-duplex interface increments the 'collisions' counter and may also record 'runts' (frames shorter than 64 bytes) due to collision fragments, while the full-duplex side may see 'frame errors' or 'CRC errors' because it receives partial or corrupted frames. Real-world scenario: a misconfigured host set to full-duplex while the switch port is set to half-duplex can cause intermittent connectivity and high error rates, often mistaken for a cable fault.
KKey Concepts to Remember
- Read the scenario before looking for a memorised answer.
- Find the constraint that changes the correct option.
- Eliminate answers that are true in general but not in this case.
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.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
How this comes up in practice
A practitioner preparing for the 200-301 exam encounters this exact type of scenario on the job. The correct answer here is not the most general option — it is the best answer for the specific constraint described. Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option. Real exam questions reward reading the full scenario before eliminating options, because the constraint defines which answer fits.
What to study next
Got this wrong? Here's your next step.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
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FAQ
Questions learners often ask
What does this 200-301 question test?
Network Infrastructure and Connectivity — This question tests Network Infrastructure and Connectivity — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Excessive collisions and CRC errors on the interface — A duplex mismatch causes collisions on the half-duplex side, resulting in excessive collisions and CRC errors (option A). On the full-duplex side, the host receives truncated frames from the half-duplex side's collisions, leading to runts and frame errors (option B). Option C is incorrect because 'Auto-negotiation failed' would appear only if negotiation itself failed, not from a mismatch after successful negotiation. Option D is wrong because a duplex mismatch typically reduces throughput and causes errors, not a high input rate. Option E is incorrect because the line protocol remains up; duplex mismatch does not bring the line protocol down.
What should I do if I get this 200-301 question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Are there clue words in this question I should notice?
Yes — watch for: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
About these practice questions
Courseiva creates original exam-style practice questions with explanations and wrong-answer analysis. It does not publish real exam questions, exam dumps, or protected exam content. Learn why practice questions differ from exam dumps →
Same concept, more angles
1 more ways this is tested on 200-301
These questions test the same concept from different angles. Work through them to make sure you can recognise it however the exam phrases it.
Variation 1. Which TWO interface issues can be identified by analyzing the output of the 'show interfaces' command?
medium- ✓ A.Duplex mismatch
- B.Routing protocol misconfiguration
- ✓ C.Speed mismatch
- D.VLAN mismatch
- E.STP topology change
Why A: The 'show interfaces' command displays interface statistics and operational status, including duplex and speed settings. A duplex mismatch occurs when one end of a link is set to full-duplex and the other to half-duplex, leading to collisions and CRC errors visible in the output. Speed mismatch is also detectable because the interface will show the negotiated speed or errors like 'input errors' if the speeds do not match.
Last reviewed: Jun 11, 2026
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