- A
Clear the MAC address table on the router to fix possible MAC flapping issues.
Why wrong: MAC flapping would cause MAC address table instability, not interface CRC errors.
- B
Administer a 'shutdown' and 'no shutdown' on the interface to reset the line protocol.
Why wrong: Resetting the interface may temporarily clear errors but won't fix the underlying hardware problem.
- C
Replace the cable with a known good one because CRC and frame errors indicate a physical layer problem such as a damaged cable or loose connector.
CRC/frame errors and carrier transitions point to physical layer issues; the recent cable change makes this likely.
- D
Change the interface speed and duplex settings to auto-negotiate because the errors are due to a mismatch between the router and CPE.
Why wrong: Speed/duplex mismatch would cause late collisions, not primarily CRC errors.
Quick Answer
The correct action is to replace the cable with a known good one because rapidly increasing CRC errors and frame errors, paired with a high number of carrier transitions, are definitive signs of a physical-layer fault. On a Juniper MX240, CRC errors indicate corrupted frames at Layer 1, while carrier transitions show the interface has repeatedly lost and regained its link signal—both pointing directly to a damaged cable or loose connector, especially since the customer’s technician recently swapped the cable. This scenario tests your ability to correlate interface counters with real-world causes, a core skill for the JNCIA-Junos exam, where a common trap is to misdiagnose CRC errors as a configuration or duplex mismatch issue when carrier transitions are present. Remember the memory tip: “CRC + carrier transitions = cable replacement mission.”
JNCIA-JUNOS Operational Monitoring and Maintenance Practice Question
This JNCIA-JUNOS practice question tests your understanding of operational monitoring and maintenance. 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.
You are managing a Juniper MX240 router at a small ISP. The router has been operating normally for months. This morning, a customer reports intermittent packet loss on their connection, which is served by interface ge-0/0/2. You SSH into the router and run 'show interfaces ge-0/0/2 extensive'. The output shows input errors increasing rapidly, including CRC errors and frame errors. The interface is up/up. You also notice that the interface statistics show a high number of carrier transitions. The cable connecting the router to the customer's CPE was recently replaced by the customer's technician. What is the most likely cause of the errors and the correct action to resolve the issue?
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 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
Replace the cable with a known good one because CRC and frame errors indicate a physical layer problem such as a damaged cable or loose connector.
CRC and frame errors increasing rapidly, combined with a high number of carrier transitions, are classic symptoms of a physical-layer issue. Since the cable was recently replaced by the customer's technician, the most likely cause is a faulty cable or a loose connector. Replacing the cable with a known good one directly addresses the physical layer problem, which is the root cause of these errors.
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.
- ✗
Clear the MAC address table on the router to fix possible MAC flapping issues.
Why it's wrong here
MAC flapping would cause MAC address table instability, not interface CRC errors.
- ✗
Administer a 'shutdown' and 'no shutdown' on the interface to reset the line protocol.
Why it's wrong here
Resetting the interface may temporarily clear errors but won't fix the underlying hardware problem.
- ✓
Replace the cable with a known good one because CRC and frame errors indicate a physical layer problem such as a damaged cable or loose connector.
Why this is correct
CRC/frame errors and carrier transitions point to physical layer issues; the recent cable change makes this likely.
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.
- ✗
Change the interface speed and duplex settings to auto-negotiate because the errors are due to a mismatch between the router and CPE.
Why it's wrong here
Speed/duplex mismatch would cause late collisions, not primarily CRC errors.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates may confuse CRC/frame errors with a speed/duplex mismatch (Option D) or think a simple interface reset (Option B) will fix the problem, when the rapid increase in carrier transitions and the recent cable replacement clearly indicate a physical cabling fault.
Detailed technical explanation
How to think about this question
CRC errors indicate that the frame check sequence (FCS) computed by the receiver does not match the transmitted FCS, typically due to signal degradation or noise on the physical medium. Frame errors occur when the entire frame is malformed, often due to alignment issues. Carrier transitions measure how many times the interface has lost and regained the carrier signal, which directly points to intermittent physical connectivity, such as a loose or damaged cable. In Juniper Junos, the 'show interfaces extensive' command provides detailed error counters and carrier transition counts, which are key diagnostic tools for isolating physical-layer faults.
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 practitioner preparing for the JNCIA-JUNOS 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
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.
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Operational Monitoring and Maintenance — study guide chapter
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FAQ
Questions learners often ask
What does this JNCIA-JUNOS question test?
Operational Monitoring and Maintenance — This question tests Operational Monitoring and Maintenance — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Replace the cable with a known good one because CRC and frame errors indicate a physical layer problem such as a damaged cable or loose connector. — CRC and frame errors increasing rapidly, combined with a high number of carrier transitions, are classic symptoms of a physical-layer issue. Since the cable was recently replaced by the customer's technician, the most likely cause is a faulty cable or a loose connector. Replacing the cable with a known good one directly addresses the physical layer problem, which is the root cause of these errors.
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.
About these practice questions
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Last reviewed: Jun 11, 2026
This JNCIA-JUNOS practice question is part of Courseiva's free Juniper Networks 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 JNCIA-JUNOS exam.
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