The answer is a duplex mismatch, specifically when the remote device is configured for half-duplex while the local interface operates in full-duplex. This is correct because high error counts—including frame check sequence errors, alignment errors, and runts—are classic symptoms of duplex mismatch high error counts troubleshooting on Ethernet interfaces. On a half-duplex link, the device waits for a clear carrier before transmitting, but a full-duplex device transmits at any time, causing collisions that corrupt frames. On the JNCIA-Junos exam, this scenario tests your ability to interpret interface error counters and recognize that modern Juniper devices default to full-duplex, so the mismatch almost always originates from a misconfigured peer. A common trap is assuming high errors indicate a cable fault, but duplex mismatch produces a distinct pattern of errors on only one side. Memory tip: think “full sends freely, half waits and collides”—if you see FCS errors and runts, check the duplex setting on the far end.
JNCIA-JUNOS User Interfaces Practice Question
This JNCIA-JUNOS practice question tests your understanding of user interfaces. 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.
Refer to the exhibit. An engineer notices high error counts on the interface. Based on the output, 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.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The interface is connected to a device configured for half-duplex.
The output shows high error counts on the interface, which is a classic symptom of a duplex mismatch. When one end of an Ethernet link operates in full-duplex and the other in half-duplex, collisions occur on the half-duplex side because it expects to wait for the carrier to be clear before transmitting, while the full-duplex side transmits at any time. This leads to frame check sequence (FCS) errors, alignment errors, and runts on the half-duplex interface. Option D is correct because the local interface is likely full-duplex (default on modern Juniper devices), and the connected device is configured for half-duplex, causing the mismatch.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
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 interface has a hardware fault.
Why it's wrong here
While possible, collisions and late collisions specifically point to a duplex mismatch.
✗
The interface is experiencing excessive broadcasts.
Why it's wrong here
Broadcast storms do not cause collisions on a full-duplex link.
✗
The interface is operating in half-duplex mode.
Why it's wrong here
The exhibit shows 'Link type: Full-Duplex'.
✓
The interface is connected to a device configured for half-duplex.
Why this is correct
Correct; collisions on a full-duplex link indicate a duplex mismatch with the remote end.
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.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often assume high error counts always indicate a hardware fault (Option A), but in JNCIA-JUNOS, the specific error types (e.g., collisions, late collisions) point to a duplex mismatch, not a physical layer failure.
Trap categories for this question
Command / output trap
The exhibit shows 'Link type: Full-Duplex'.
Detailed technical explanation
How to think about this question
In Ethernet, duplex mismatch occurs when one side uses IEEE 802.3x flow control (full-duplex) and the other uses CSMA/CD (half-duplex). The half-duplex side detects collisions and retransmits, but the full-duplex side does not back off, leading to late collisions and excessive deferrals. Juniper devices show these errors in the 'show interfaces extensive' output under 'Input errors' and 'Output errors', with specific counters like 'Collisions' and 'Late collisions' being key indicators of a mismatch.
KKey Concepts to Remember
Read the scenario before looking for a memorised answer.
Find the constraint that changes the correct option.
Eliminate answers that are true in general but not in this case.
TExam Day Tips
→Watch for words such as best, first, most likely and least administrative effort.
→Review why wrong options are wrong, not only why the correct option is correct.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
How this comes up in practice
A network engineer segments a warehouse floor into three subnets: 20 scanners, 5 printers, and 2 management hosts. Picking the wrong mask wastes addresses or leaves too few usable hosts. Exam questions test whether you can apply CIDR notation, calculate block size, and identify the correct usable-host range for a given prefix.
What to study next
Got this wrong? Here's your next step.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
User Interfaces — This question tests User Interfaces — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The interface is connected to a device configured for half-duplex. — The output shows high error counts on the interface, which is a classic symptom of a duplex mismatch. When one end of an Ethernet link operates in full-duplex and the other in half-duplex, collisions occur on the half-duplex side because it expects to wait for the carrier to be clear before transmitting, while the full-duplex side transmits at any time. This leads to frame check sequence (FCS) errors, alignment errors, and runts on the half-duplex interface. Option D is correct because the local interface is likely full-duplex (default on modern Juniper devices), and the connected device is configured for half-duplex, causing the mismatch.
What should I do if I get this JNCIA-JUNOS question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Are there clue words in this question I should notice?
Yes — watch for: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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Question Discussion
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