- A
Runts
Runts are frames smaller than 64 bytes, often caused by collisions on a half-duplex link due to a duplex mismatch.
- B
Giants
Why wrong: Giants are frames larger than 1518 bytes, typically due to MTU misconfiguration or faulty hardware, not duplex mismatch.
- C
CRC errors
CRC errors occur when a frame's cyclic redundancy check fails, often from collisions or noise on a half-duplex link caused by duplex mismatch.
- D
Input errors
Why wrong: Input errors is a general category that includes runts, CRC errors, and others; it is not a specific error type directly tied to duplex mismatch.
- E
Output errors
Why wrong: Output errors include collisions, late collisions, and other issues; though collisions can occur, output errors are not exclusively caused by duplex mismatch.
- F
Flaps
Why wrong: Flaps refer to the interface repeatedly going up and down, usually due to physical issues like loose cables or faulty transceivers, not duplex mismatch.
Duplex Mismatch — Runts and CRC Errors on Switch Interfaces
This 200-301 practice question tests your understanding of network infrastructure and connectivity. Read the scenario carefully and evaluate each option against the stated constraints before committing to an answer. 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 interface errors are most likely caused by a mismatch in duplex settings between two connected switches?
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.
Quick Answer
The answer is CRC errors and runts. This happens because a duplex mismatch forces one switch to operate at half duplex while the other runs at full duplex; on the half-duplex side, the interface treats incoming frames as collisions during transmission, truncating them into fragments under 64 bytes known as runts, while the full-duplex side receives these corrupted frames without detecting collisions, causing both runts and CRC errors to spike as the truncated data fails the Frame Check Sequence validation. On the CCNA 200-301 v2 exam, this topic tests your ability to diagnose physical-layer issues from interface counters, often appearing in troubleshooting scenarios where you must correlate error types with misconfigured duplex settings—a common trap is assuming CRC errors alone point to cabling faults. Remember the memory tip: half-duplex sees collisions and makes runts; full-duplex sees runts and logs CRC errors.
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
Runts
A duplex mismatch occurs when one switch operates at full duplex while the other operates at half duplex. On the half-duplex side, frames arriving while the interface is transmitting are considered collisions, causing the frame to be truncated into fragments (runts). On the full-duplex side, the switch does not detect collisions but may receive incomplete frames, which are counted as runts if they are less than 64 bytes. CRC errors also spike because the truncated or corrupted frames fail the Frame Check Sequence (FCS) validation.
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.
- ✓
Runts
Why this is correct
Runts are frames smaller than 64 bytes, often caused by collisions on a half-duplex link due to a duplex mismatch.
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.
- ✗
Giants
Why it's wrong here
Giants are frames larger than 1518 bytes, typically due to MTU misconfiguration or faulty hardware, not duplex mismatch.
- ✓
CRC errors
Why this is correct
CRC errors occur when a frame's cyclic redundancy check fails, often from collisions or noise on a half-duplex link caused by duplex mismatch.
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.
- ✗
Input errors
Why it's wrong here
Input errors is a general category that includes runts, CRC errors, and others; it is not a specific error type directly tied to duplex mismatch.
- ✗
Output errors
Why it's wrong here
Output errors include collisions, late collisions, and other issues; though collisions can occur, output errors are not exclusively caused by duplex mismatch.
- ✗
Flaps
Why it's wrong here
Flaps refer to the interface repeatedly going up and down, usually due to physical issues like loose cables or faulty transceivers, not duplex mismatch.
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.
✓RuntsCorrect answer▾
Why this is correct
Runts are frames smaller than 64 bytes, often caused by collisions on a half-duplex link due to a duplex mismatch.
✗GiantsWrong answer — click to see why▾
Why this is wrong here
Giants are frames exceeding the maximum size (typically 1518 bytes) and are caused by MTU misconfiguration, faulty NICs, or software errors, not by duplex mismatch. Duplex mismatch does not affect frame size; it causes collisions and CRC errors.
Why candidates choose this
Students might confuse giants with runts, thinking both are size-related errors caused by duplex issues. However, giants are associated with oversized frames, not collisions.
✗Input errorsWrong answer — click to see why▾
Why this is wrong here
Input errors is a broad counter that includes runts, CRC errors, frame errors, and others. While duplex mismatch can contribute to some input errors, it is not a specific error type. The question asks for 'interface errors' most likely caused by duplex mismatch, and input errors is too generic.
Why candidates choose this
Since runts and CRC errors are input errors, a test-taker might think 'input errors' is a direct answer. However, the question expects specific error types, not a category.
✗Output errorsWrong answer — click to see why▾
Why this is wrong here
Output errors include collisions, late collisions, and underruns. While collisions can occur due to duplex mismatch, output errors are not exclusively caused by duplex mismatch; they can result from other issues like cable faults or interface congestion. The question asks for errors 'most likely' caused by duplex mismatch, and runts and CRC errors are more directly linked.
Why candidates choose this
Collisions are a known symptom of duplex mismatch, and collisions are counted as output errors. However, the presence of collisions alone does not make 'output errors' the best answer, as the question requires two specific errors.
✗FlapsWrong answer — click to see why▾
Why this is wrong here
Flaps refer to an interface repeatedly going up and down, typically due to physical layer issues like loose cables, faulty transceivers, or power fluctuations. Duplex mismatch does not cause interface flaps; it causes errors on the link but the interface remains up.
Why candidates choose this
Students might think that any interface problem is a 'flap', but flaps are specifically about link state changes, not error counters. The term 'flap' is often misused in casual conversation.
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 between runts and giants, where candidates mistakenly think giants are caused by duplex mismatch, but giants are actually linked to jumbo frames or faulty hardware, not duplex negotiation issues.
Trap categories for this question
Command / output trap
Output errors include collisions, late collisions, and other issues; though collisions can occur, output errors are not exclusively caused by duplex mismatch.
Detailed technical explanation
How to think about this question
When a full-duplex interface transmits a frame while the half-duplex peer is still sending, the half-duplex side sees a collision and performs a backoff algorithm (CSMA/CD), but the full-duplex side does not retransmit, leading to a partial frame. The half-duplex interface then receives a fragment (runt) and may also report a late collision if the collision occurs after the first 64 bytes. On the full-duplex side, the incomplete frame fails the CRC check, incrementing CRC errors. The 'show interfaces' command on Cisco switches displays these counters, and a rapidly increasing 'runts' counter alongside 'CRC' errors is a classic symptom of duplex mismatch.
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
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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: Runts — A duplex mismatch occurs when one switch operates at full duplex while the other operates at half duplex. On the half-duplex side, frames arriving while the interface is transmitting are considered collisions, causing the frame to be truncated into fragments (runts). On the full-duplex side, the switch does not detect collisions but may receive incomplete frames, which are counted as runts if they are less than 64 bytes. CRC errors also spike because the truncated or corrupted frames fail the Frame Check Sequence (FCS) validation.
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.
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Last reviewed: Jun 11, 2026
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