220-1101 Networking Practice Question
A technician is troubleshooting a user's workstation that has intermittent network connectivity. The technician observes that the Ethernet link light on the workstation's NIC flashes irregularly and occasionally stays off for a few seconds. The cable passes a wiremap test. The switch port is configured for auto-negotiation. Which of the following is the MOST likely cause?
⚠ Common exam trap
The trap here is that candidates see a passing wiremap test and assume the cable is perfect, overlooking that EMI can corrupt signals on electrically intact cables, and they often confuse Layer 1 physical issues (link light behavior) with Layer 2/3 problems like IP conflicts or driver faults.
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
✓
Electromagnetic interference from nearby power cables.
The intermittent flashing and occasional off periods of the Ethernet link light, combined with a passing wiremap test, strongly indicate that the signal is being corrupted by external interference. Electromagnetic interference (EMI) from nearby power cables can disrupt the differential signaling on twisted-pair copper cabling, causing link drops and flapping link lights even when the physical wiring is intact. This matches the symptom of a link light that flashes irregularly and stays off for seconds, as the NIC and switch repeatedly lose and re-establish the link due to noise.
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 is too long (exceeds 100 meters).
Why it's wrong here
Ethernet over twisted pair (10/100/1000BASE-T) is specified for a maximum segment length of 100 meters; exceeding this causes excessive attenuation and signal-timing degradation, which usually results in either complete loss of link or a continuous stream of FCS/CRC errors. This would not present as intermittent link-light flickering because the physical layer remains consistently unusable or borderline, not cyclically healthy and unhealthy. The cable’s length is a design constraint that typically produces a stable, severe failure rather than EMI-induced temporary dropouts.
- ✗
The workstation's IP address is a duplicate.
Why it's wrong here
A duplicate IP address is a Layer 3 (network-layer) conflict that occurs between two hosts using the same logical address, causing ARP fluctuations and intermittent unreachability or dropped traffic. However, the link-light LED on the NIC is a Layer 1 (physical-layer) indicator that reflects only the presence of a valid electrical/training signal on the wire, independent of IP or other upper-layer configuration. Thus, a duplicate IP can disrupt connectivity at the protocol level while the link light remains steadily illuminated.
- ✓
Electromagnetic interference from nearby power cables.
Why this is correct
Electromagnetic interference (EMI) from nearby AC power cables, motors, or fluorescent fixtures injects common-mode noise into an unshielded twisted-pair cable; when the noise amplitude exceeds the differential-mode rejection capability of the transceiver, corrupted link pulses and data frames appear. This can cause intermittent link loss during bursts of interference while the link appears normal when the interference is absent. The link light mimics these dropouts because the physical layer loses its ability to maintain the link-train handshake, and shielding, rerouting, or increasing cable category can restore stable operation.
- ✗
The NIC driver is out of date.
Why it's wrong here
An out-of-date NIC driver is a software component that operates at Layers 2–3 (and above) to manage the interface's configuration and data buffers; it never directly controls the electrical state of the physical transceiver that drives the link LED. Even if the driver crashes or is uninstalled, the NIC's hardware maintains link status and the link light stays lit, provided the physical connection is intact. Driver issues instead manifest as error events, low throughput, or inability to assign an IP address, rather than a visible loss of physical link.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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