CCNA Network Infrastructure and Connectivity Practice Question
Which TWO interface errors are most likely caused by a mismatch in duplex settings between two connected switches?
⚠ Common exam trap
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.
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.
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 Ethernet frames smaller than the minimum 64-byte size, and they are a classic symptom of duplex mismatch. When one end operates at half duplex and the other at full duplex, the half-duplex end may sense a collision (or late collision) and abort its transmission prematurely, leaving a truncated frame that the full-duplex receiver counts as a runt. Thus, a high rate of runts on an interface strongly suggests that the peer is running at a different duplex setting than the local switch port.
- ✗
Giants
Why it's wrong here
Giants are frames exceeding the 1518-byte maximum size, typically reflecting an MTU misconfiguration, an enabling of jumbo frames across a path, or faulty hardware that fails to enforce the frame-size limit. A duplex mismatch does not produce oversized frames; the resulting collisions and backoff logic cause short transmissions or bit errors, not extended-length buffers. Consequently, a network administrator seeing giants should investigate layer-2 MTU policies or NIC failures rather than suspecting a duplex negotiation problem.
- ✓
CRC errors
Why this is correct
CRC (cyclic redundancy check) errors occur when the frame's checksum does not match the transmitted data, indicating bit-level corruption during delivery. In a duplex mismatch scenario, the half-duplex station's late collisions can corrupt frames in transit; these frames may still meet the minimum length but fail the CRC verification at the receiving interface. While runts represent truncated frames, CRC errors specifically flag integrity failures in frames that have arrived (possibly complete in length but damaged), making both counters complementary evidence of a duplex mismatch.
- ✗
Input errors
Why it's wrong here
Input errors is a cumulative counter that aggregates specific inbound error types such as runts, CRC errors, and frame errors. A duplex mismatch typically manifests as runts or CRC errors on the receiving interface, so merely seeing a positive input errors count does not isolate the cause. Network engineers must inspect the detailed subcounters to identify the exact frame-level error, making 'input errors' too generic for this specific failure mode.
- ✗
Output errors
Why it's wrong here
Output errors is a broad counter encompassing transmit-side anomalies such as collisions, late collisions, underruns, and buffer failures, so it can increase for many reasons unrelated to duplex mismatch. On a half-duplex link experiencing a mismatch, the interface will indeed log collisions (a subset of output errors), but the more specific late-collision counter is the decisive indicator. Therefore, while output errors may be elevated, they are not a specific diagnostic marker unless the late-collision count is examined separately, making the option too general for the scenario.
- ✗
Flaps
Why it's wrong here
Interface flaps refer to a link that repeatedly transitions between up and down states, often triggered by physical-layer problems like a loose cable, faulty optics, or excessive signal loss. A duplex mismatch does not cause the link to go down; instead, the link remains up while suffering from collision-induced corruption and timing issues. Therefore, flaps are attributable to physical integrity problems rather than to mismatched duplex settings between the two connected devices.
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 Ethernet frames smaller than the minimum 64-byte size, and they are a classic symptom of duplex mismatch. When one end operates at half duplex and the other at full duplex, the half-duplex end may sense a collision (or late collision) and abort its transmission prematurely, leaving a truncated frame that the full-duplex receiver counts as a runt. Thus, a high rate of runts on an interface strongly suggests that the peer is running at a different duplex setting than the local switch port.
✗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?”
Go deeper
Related to this question
Learn chapter
Half-Duplex vs Full-Duplex
Key term
Half duplex
Half duplex is a data transmission mode that allows communication in both directions, but only one direction at a time, like a walkie-talkie.
Key term
Frame
A frame is a unit of data at the Data Link layer of networking, containing header, payload, and trailer information for local network delivery.
About these practice questions
One of 1,389 original 200-301 practice questions on Courseiva, each with a full explanation and wrong-answer analysis — not exam dumps or protected exam content. Learn why practice questions differ from exam dumps →
JA
Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This 200-301 practice question is part of Courseiva's free Cisco certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the 200-301 exam.