220-1101 Networking Practice Question
A small office network consists of a router and an unmanaged switch. Every day at 2:00 PM, users report extremely slow network performance and intermittent connectivity that lasts about 30 minutes. A technician checks the switch's interface statistics and notices a high number of CRC errors on the port connected to a VoIP phone. The cable connecting the phone to the wall jack is a known working Cat5e cable. Which of the following is the MOST likely cause of the issue?
⚠ Common exam trap
CompTIA often tests duplex mismatch by pairing CRC errors with a specific time-based pattern, tricking candidates into blaming the cable or broadcast storms instead of recognizing that CRC errors on a single port point to a layer 1/2 issue like duplex 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 VoIP phone is configured for half-duplex while the switch port is using full-duplex.
A is correct because a duplex mismatch between the VoIP phone (half-duplex) and the switch port (full-duplex) causes CRC errors, collisions, and performance degradation. The daily 2:00 PM timing suggests a scheduled task like a phone firmware update or DHCP renewal that triggers heavy traffic, exacerbating the mismatch. CRC errors specifically indicate layer 1/2 frame corruption, which is a classic symptom of duplex mismatch.
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 VoIP phone is configured for half-duplex while the switch port is using full-duplex.
Why this is correct
A duplex mismatch occurs when one device operates in half-duplex and the other in full-duplex. In this scenario, the VoIP phone's manual half-duplex setting overrides auto-negotiation, while the switch port is forced to full-duplex. The half-duplex device fails to properly detect collisions, leading to late collisions and cyclic redundancy check (CRC) errors on the switch port, which manifests as slow performance and corrupted frames. This is a classic and common misconfiguration, especially with VoIP phones that have hard-coded NIC settings.
- ✗
The switch is experiencing a broadcast storm caused by a loop in the network.
Why it's wrong here
A broadcast storm caused by a Layer 2 loop would flood all switch ports with repeated broadcast frames, consuming the CPU and saturating the entire network, not just one port. CRC errors on a single switch port are not a symptom of a broadcast storm; instead, you would see high overall utilization, broadcast traffic counts spiking on all ports, and possibly a loop detection protocol (like Spanning Tree) disabling ports. The problem described is isolated to one device, pointing to a physical or duplex issue rather than a network-wide loop.
- ✗
The VoIP phone is running QoS that priority-tags all traffic.
Why it's wrong here
QoS priority tagging, such as 802.1p, modifies the Priority Code Point (PCP) bits in the VLAN tag of Ethernet frames. This process does not alter the frame payload or include any mechanism that would introduce bit errors or cause CRC failures. While QoS can affect traffic scheduling and prioritization, it cannot corrupt frames or produce CRC errors, nor does it inherently slow down the network; any perceived slowdown from QoS is due to queuing policies, not frame corruption.
- ✗
The Category 5e cable is not capable of handling VoIP traffic.
Why it's wrong here
Category 5e (Cat5e) cable is fully rated for 1000BASE-T (Gigabit Ethernet) up to 100 meters, and VoIP traffic typically requires only about 100 kbps per call. The bandwidth demands of VoIP are minuscule compared to what Cat5e can support, so the cable is not a limiting factor. If the cable were faulty or poorly terminated, you might see CRC errors or link drops, but the cable's category alone does not preclude VoIP operation.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This 220-1101 practice question is part of Courseiva's free CompTIA 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 220-1101 exam.