CCNA Network Infrastructure and Connectivity Practice Question
Which TWO statements correctly describe interface errors and duplex mismatches on Cisco switches?
⚠ Common exam trap
Cisco often tests the distinction between late collisions (which occur after the 64-byte window and indicate a duplex mismatch) and early collisions (which occur within the window and are normal in half-duplex), and candidates mistakenly think that all collisions are normal or that CRC errors are the primary cause of runts.
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
✓
Late collisions on a half-duplex interface can indicate a duplex mismatch with the connected device.
Late collisions occur when a frame is transmitted onto the wire and collides after the first 512 bit-times of the frame. In a half-duplex Ethernet segment, a duplex mismatch causes the full-duplex side to never defer and transmit at any time, while the half-duplex side expects to detect collisions only during the collision window. When the full-duplex device sends a frame while the half-duplex device is already transmitting, the half-duplex device detects a collision after the 512-bit window, resulting in a late collision.
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 are typically caused by CRC errors on the transmitting device.
Why it's wrong here
Runts are frames shorter than the 64-byte Ethernet minimum, and they are typically the result of collisions or faulty hardware, not CRC errors. CRC errors indicate that a received frame has bit corruption, which is a different failure mode. While both appear as input errors, runts and CRCs are independently counted and must be analyzed separately for interface health.
- ✓
Late collisions on a half-duplex interface can indicate a duplex mismatch with the connected device.
Why this is correct
In half-duplex Ethernet, a late collision occurs after the first 512 bits have been transmitted, meaning the sending station has already finished its collision window. When one device is full-duplex and the other half-duplex, the full-duplex side never defers to traffic, transmitting while the half-duplex side is sending, causing a collision that the half-duplex side detects too late. This timing signature is a classic indicator of a duplex mismatch.
- ✓
The 'show interfaces' command displays the number of CRC errors and runts on an interface.
Why this is correct
The 'show interfaces' command on Cisco IOS outputs detailed interface counters, including a specific line for input errors that lists runts, giants, CRC errors, frame, overrun, and ignored counts. By examining these fields, a network engineer can quickly identify physical-layer or duplexing issues without additional packet captures. This makes it an essential first step in troubleshooting Ethernet link reliability.
- ✗
Full-duplex interfaces use CSMA/CD to detect collisions before transmitting.
Why it's wrong here
Full-duplex Ethernet links are point-to-point connections where each station has its own dedicated transmit and receive path, so there is no shared medium to contend for. Because of this, CSMA/CD is completely disabled in full-duplex mode; the interface neither listens before transmitting nor detects collisions. Collisions simply cannot occur on a properly configured full-duplex link, as frames are sent and received simultaneously.
- ✗
Auto-MDIX can resolve a duplex mismatch by renegotiating the speed and duplex settings.
Why it's wrong here
Auto-MDIX is a physical-layer feature that automatically swaps the transmit and receive pairs in a twisted-pair cable, allowing a straight-through or crossover cable to be used interchangeably. It does not participate in speed or duplex negotiation, which is handled separately by the auto-negotiation process defined in IEEE 802.3. Therefore, Auto-MDIX cannot fix a duplex mismatch; it only corrects cabling pair polarity.
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.
✓Late collisions on a half-duplex interface can indicate a duplex mismatch with the connected device.Correct answer▾
Why this is correct
In half-duplex Ethernet, a late collision occurs after the first 512 bits have been transmitted, meaning the sending station has already finished its collision window. When one device is full-duplex and the other half-duplex, the full-duplex side never defers to traffic, transmitting while the half-duplex side is sending, causing a collision that the half-duplex side detects too late. This timing signature is a classic indicator of a duplex mismatch.
✗Runts are typically caused by CRC errors on the transmitting device.Wrong answer — click to see why▾
Why this is wrong here
Runts result from collisions on half-duplex links or faulty network interface cards, not from CRC errors.
✗Full-duplex interfaces use CSMA/CD to detect collisions before transmitting.Wrong answer — click to see why▾
Why this is wrong here
CSMA/CD is only used in half-duplex environments; full-duplex disables collision detection entirely.
✗Auto-MDIX can resolve a duplex mismatch by renegotiating the speed and duplex settings.Wrong answer — click to see why▾
Why this is wrong here
Auto-MDIX does not participate in speed or duplex negotiation; that function is handled by auto-negotiation (IEEE 802.3u).
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
Collision Domains and Broadcast Domains
Key term
Ethernet
Ethernet is a wired networking technology that connects devices like computers, printers, and servers to each other and to the internet using cables.
Key term
Bit
A bit is the smallest unit of data in computing and digital communications, representing a single binary value of either 0 or 1.
About these practice questions
Courseiva writes every 200-301 question from scratch — 1,389 in total, each with an explanation and a wrong-answer breakdown. None are copied from real exams or dumps. 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.