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350-401 Practice Question: Is troubleshooting a wireless network where…
A network engineer is troubleshooting a wireless network where clients in a conference room experience intermittent connectivity. The engineer notices that the access point in that room is showing a high number of CRC errors on its uplink interface. The AP is connected to a Cisco 9300 switch via a copper cable. What is the most likely cause of the CRC errors?
⚠ Common exam trap
Cisco often tests the distinction between CRC errors (physical layer) and duplex-mismatch errors (runts/giants/FCS), leading candidates to incorrectly choose duplex mismatch when the symptom is specifically 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
✓
The Ethernet cable is faulty or of poor quality.
CRC errors on an Ethernet interface indicate that frames are being received with invalid checksums, typically due to physical-layer issues such as signal degradation, crosstalk, or faulty cabling. Since the AP is connected via a copper cable to a Cisco 9300 switch, a faulty or poor-quality Ethernet cable is the most direct cause of these errors, as it can introduce bit errors that corrupt the frame's FCS.
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 AP is overloaded with too many clients.
Why it's wrong here
Client overload is an RF-layer capacity problem: too many associated clients contend for airtime, causing throughput degradation, retransmissions, and latency, but it does not affect the integrity of the Ethernet frames traversing the wired uplink. CRC errors are computed at the physical layer of the copper link, so the AP's client load is irrelevant to frame checksum failures.
- ✓
The Ethernet cable is faulty or of poor quality.
Why this is correct
CRC errors on a copper Ethernet link indicate that the receiving interface detected frames whose cyclic redundancy check did not match the computed value, which is almost always caused by electrical signal corruption. A faulty or low-quality cable (e.g., damaged pairs, improper termination, or crosstalk) introduces bit errors that corrupt the frame at the physical layer. This directly explains why the AP's wired uplink shows CRC errors while the rest of the switch port statistics appear normal.
- ✗
The switch port is configured with a duplex mismatch.
Why it's wrong here
Duplex mismatch on an Ethernet link typically produces a specific signature of errors: late collisions, runts, giants, and FCS/CRC errors, but it also generates excessive alignment errors and very high collision counts. While CRC errors can appear as a secondary symptom, the primary indicator is a massive number of runts and alignment errors on the half-duplex side. Since the question emphasizes CRC errors without mentioning these classic symptoms, a duplex mismatch is an incomplete and less precise explanation than a faulty cable.
- ✗
The AP is not receiving enough power from Power over Ethernet (PoE).
Why it's wrong here
Insufficient PoE power does not cause CRC errors on the wired interface; instead, the AP will either fail to boot, continuously reboot, or experience brownouts that cause the radio and Ethernet controller to reset. When the AP does manage to operate, the uplink either goes down or recovers, but the frames that do pass are not corrupted. CRC errors are a physical-layer phenomenon of the cable medium, not a power-delivery issue, so this option is incorrect.
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JA
Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This 350-401 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 350-401 exam.