CCNA Network Infrastructure and Connectivity Practice Question
A network administrator notices that file transfers to a server are extremely slow, and on the switch interface connecting to the server, the output of 'show interfaces' indicates a high number of runts and CRC errors, but no collisions. Which of the following is the most likely cause?
⚠ Common exam trap
Cisco often tests the distinction between symptoms of duplex mismatch versus cable faults, where candidates mistakenly attribute runts and CRC errors to a bad cable, ignoring the critical clue of zero collisions that points to a mismatch.
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
✓
The switch port is configured for full-duplex, but the server's NIC is set to half-duplex.
The combination of runts (frames smaller than 64 bytes) and CRC errors with zero collisions is a classic symptom of a duplex mismatch. When one side operates at full-duplex and the other at half-duplex, the half-duplex side will detect collisions and invoke its backoff algorithm, causing the full-duplex side to receive truncated frames (runts) and frames with invalid FCS (CRC errors). The switch interface statistics show no collisions because the switch port is full-duplex and does not detect collisions, while the server's half-duplex NIC is causing the corruption.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
The cable connecting the server to the switch is faulty.
Why it's wrong here
A faulty cable can cause CRC errors, but typically also causes input errors, and may still experience collisions if both sides are half-duplex. It does not specifically explain the presence of runts alongside CRC errors without collisions.
- ✓
The switch port is configured for full-duplex, but the server's NIC is set to half-duplex.
Why this is correct
A duplex mismatch where the switch uses full-duplex and the server uses half-duplex results in the full-duplex side (switch) showing runts and CRC errors without collisions, while the half-duplex side sees collisions. This matches the 'show interfaces' output and explains the slow transfers due to excessive retransmissions.
- ✗
The switch port speed is set to 100 Mbps, but the server NIC is set to 10 Mbps.
Why it's wrong here
A hard-set speed mismatch (100 vs 10) usually results in the link not coming up because both sides cannot establish a valid connection. The interfaces would be in a down/down state, not forwarding traffic, so file transfers would be impossible, not just slow.
- ✗
The server's NIC driver is outdated, causing packet loss.
Why it's wrong here
An outdated driver could lead to performance issues or packet loss, but it would not selectively generate runts and CRC errors on the switch port. Those errors indicate Layer 1 or Layer 2 issues on the link, typically related to physical layer problems or duplex mismatches.
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.
✓The switch port is configured for full-duplex, but the server's NIC is set to half-duplex.Correct answer▾
Why this is correct
A duplex mismatch where the switch uses full-duplex and the server uses half-duplex results in the full-duplex side (switch) showing runts and CRC errors without collisions, while the half-duplex side sees collisions. This matches the 'show interfaces' output and explains the slow transfers due to excessive retransmissions.
✗The cable connecting the server to the switch is faulty.Wrong answer — click to see why▾
Why this is wrong here
While a faulty cable could cause CRC errors, it would likely produce other error types and might not exhibit the specific pattern of only runts and CRC errors with no collisions. This pattern strongly points to a duplex mismatch.
✗The switch port speed is set to 100 Mbps, but the server NIC is set to 10 Mbps.Wrong answer — click to see why▾
Why this is wrong here
A speed mismatch would generally cause the link to fail entirely; you wouldn't see interface errors because there would be no connectivity. The scenario describes connectivity with errors, so this is unlikely.
✗The server's NIC driver is outdated, causing packet loss.Wrong answer — click to see why▾
Why this is wrong here
Outdated drivers may cause performance problems, but they do not produce the specific interface error counters on the switch. The recorded runts and CRC errors point to a physical or data-link layer issue, not a driver problem.
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
Network Topology Types
Key term
Switch
A switch is a networking device that connects devices on a local area network and uses MAC addresses to forward data only to the intended recipient.
Key term
Duplex mismatch
A duplex mismatch occurs when two connected network devices are configured with different duplex settings (one half-duplex, one full-duplex), leading to poor performance and data errors.
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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.