Quick Answer
The answer is a bad Ethernet cable, as the intermittent connectivity with the link light on but the activity light flashing rapidly and irregularly is a classic symptom of a faulty or marginal cable. This occurs because a damaged cable introduces electrical noise or breaks in the conductors, causing frame errors and excessive collisions that force the network interface to retransmit data, which the activity light reflects as rapid, erratic flashes while the physical link remains detected. On the CompTIA ITF+ FC0-U61 exam, this scenario tests your ability to distinguish physical layer issues from software or configuration problems—a common trap is to blame IP conflicts or driver corruption, which would not produce this specific link/activity light behavior. For a memory tip, remember that a steady link light means the cable is physically connected, but a wild, flickering activity light signals a bad cable causing data chaos.
This FC0-U61 practice question tests your understanding of infrastructure. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
After a network upgrade, a user reports that their computer intermittently loses network connectivity. The link light remains on, but the activity light flashes rapidly and irregularly. What is the most likely cause?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "most likely"
Why it matters: Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
IP address conflict
Why wrong: An IP conflict would likely cause continuous disconnection or error messages.
Bad Ethernet cable
A bad cable can cause intermittent connectivity and rapid error flashes.
Driver corruption
Why wrong: Driver issues usually cause persistent problems, not intermittent with link light on.
Faulty NIC
Why wrong: A faulty NIC typically causes the link light to be off or intermittent, not on solid.
Answer choices
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
Bad Ethernet cable
The intermittent connectivity with the link light on but the activity light flashing rapidly and irregularly is a classic symptom of a faulty or marginal Ethernet cable. A bad cable can cause excessive collisions or frame errors, leading to retransmissions that manifest as rapid, irregular activity light flashing while the physical link (link light) remains established. This is distinct from issues like IP conflicts or driver corruption, which typically do not produce this specific link/activity light behavior.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
For each option: why learners choose it and why it is or isn't the right answer here.
IP address conflict
Why it's wrong here
An IP conflict would likely cause continuous disconnection or error messages.
Bad Ethernet cable
Why this is correct
A bad cable can cause intermittent connectivity and rapid error flashes.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
Driver corruption
Why it's wrong here
Driver issues usually cause persistent problems, not intermittent with link light on.
Faulty NIC
Why it's wrong here
A faulty NIC typically causes the link light to be off or intermittent, not on solid.
Common exam traps
The trap here is that candidates often confuse physical layer symptoms (bad cable) with Layer 2 or Layer 3 issues (IP conflict, driver corruption), assuming that intermittent connectivity must be a software or configuration problem rather than a hardware fault.
Detailed technical explanation
Ethernet cables use twisted pairs to cancel electromagnetic interference; a damaged cable can introduce crosstalk or impedance mismatches, causing frame check sequence (FCS) errors and late collisions. The rapid, irregular activity light indicates the NIC is attempting to retransmit corrupted frames, a behavior that can be confirmed by checking interface error counters (e.g., 'show interface' for CRC errors or collisions). In real-world scenarios, a cable that passes a basic continuity test may still fail under higher data rates (e.g., Gigabit Ethernet) due to signal degradation.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
A practitioner preparing for the FC0-U61 exam encounters this exact type of scenario on the job. The correct answer here is not the most general option — it is the best answer for the specific constraint described. Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option. Real exam questions reward reading the full scenario before eliminating options, because the constraint defines which answer fits.
What to study next
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FAQ
Infrastructure — This question tests Infrastructure — Read the scenario before looking for a memorised answer..
The correct answer is: Bad Ethernet cable — The intermittent connectivity with the link light on but the activity light flashing rapidly and irregularly is a classic symptom of a faulty or marginal Ethernet cable. A bad cable can cause excessive collisions or frame errors, leading to retransmissions that manifest as rapid, irregular activity light flashing while the physical link (link light) remains established. This is distinct from issues like IP conflicts or driver corruption, which typically do not produce this specific link/activity light behavior.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Yes — watch for: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
Read the scenario before looking for a memorised answer.
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Last reviewed: Jun 25, 2026
This FC0-U61 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 FC0-U61 exam.
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