The answer is a faulty SFP module. This is the most likely cause of interface flapping because the log messages show repeated link up and down events without any protocol-level errors, which points directly to a physical layer issue rather than a configuration mismatch. In Junos, degraded optics or loose SFP connections trigger continuous link transitions, making the SFP module the prime suspect when troubleshooting interface flapping. On the JNCIA-Junos exam, this scenario tests your ability to differentiate hardware faults from Layer 2 or Layer 3 problems—a common trap is to blame duplex or MTU mismatches, but those usually generate error counters or negotiation failures, not clean link flaps. A useful memory tip: “If the link dances but the logs are clean, suspect the SFP machine.”
JNCIA-JUNOS Operational Monitoring and Maintenance Practice Question
This JNCIA-JUNOS practice question tests your understanding of operational monitoring and maintenance. Examine the command output carefully: the correct answer depends on what the output actually shows, not on general recall alone. 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.
Exhibit
Output of 'show log messages | last 10':
Feb 15 10:00:00 router rpd[1234]: RPD_INTERFACE_DOWN: Interface ge-0/0/0.0 changed state to Down
Feb 15 10:00:01 router rpd[1234]: RPD_INTERFACE_UP: Interface ge-0/0/0.0 changed state to Up
Feb 15 10:00:02 router rpd[1234]: RPD_INTERFACE_DOWN: Interface ge-0/0/0.0 changed state to Down
Feb 15 10:00:03 router rpd[1234]: RPD_INTERFACE_UP: Interface ge-0/0/0.0 changed state to Up
(repeated every second)
Refer to the exhibit. Based on the log messages, what is the most likely cause of the interface flapping?
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.
Output of 'show log messages | last 10':
Feb 15 10:00:00 router rpd[1234]: RPD_INTERFACE_DOWN: Interface ge-0/0/0.0 changed state to Down
Feb 15 10:00:01 router rpd[1234]: RPD_INTERFACE_UP: Interface ge-0/0/0.0 changed state to Up
Feb 15 10:00:02 router rpd[1234]: RPD_INTERFACE_DOWN: Interface ge-0/0/0.0 changed state to Down
Feb 15 10:00:03 router rpd[1234]: RPD_INTERFACE_UP: Interface ge-0/0/0.0 changed state to Up
(repeated every second)
A
Faulty SFP module
Rapid link up/down events (flapping) are often caused by physical layer issues such as a faulty SFP, loose cable, or bad fiber. This pattern is characteristic of hardware failure.
B
MTU mismatch
Why wrong: MTU mismatch causes packet drops, not link state changes. It would not cause the interface to go down and up.
C
Duplicate IP address
Why wrong: A duplicate IP address would cause address conflict messages but not rapid link state changes. It would not cause the interface to go down and up repeatedly.
D
Spanning tree topology change
Why wrong: Spanning tree changes cause interface state transitions (blocking/forwarding) but not such rapid toggling between up and down. STP events are typically spaced by seconds.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Faulty SFP module
The log messages show repeated link up/down events (interface flapping). In Junos, interface flapping is most commonly caused by a faulty SFP module, as physical layer issues like degraded optics or loose connections trigger continuous link transitions. The logs typically show 'link up' followed by 'link down' without any protocol-level errors, pointing to a hardware fault rather than configuration mismatches.
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.
✓
Faulty SFP module
Why this is correct
Rapid link up/down events (flapping) are often caused by physical layer issues such as a faulty SFP, loose cable, or bad fiber. This pattern is characteristic of hardware failure.
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.
✗
MTU mismatch
Why it's wrong here
MTU mismatch causes packet drops, not link state changes. It would not cause the interface to go down and up.
✗
Duplicate IP address
Why it's wrong here
A duplicate IP address would cause address conflict messages but not rapid link state changes. It would not cause the interface to go down and up repeatedly.
✗
Spanning tree topology change
Why it's wrong here
Spanning tree changes cause interface state transitions (blocking/forwarding) but not such rapid toggling between up and down. STP events are typically spaced by seconds.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often attribute interface flapping to higher-layer issues like MTU or IP conflicts, but Junos logs physical link transitions (up/down) are almost always due to Layer 1 problems such as faulty optics, cables, or hardware, not configuration mismatches.
Detailed technical explanation
How to think about this question
Interface flapping due to faulty SFP modules often involves intermittent electrical or optical signal loss, which triggers the Junos 'link-state' process to deassert and reassert carrier detect. This can be diagnosed using 'show interfaces diagnostics optics' to check for high bit error rates or low receive power, and 'show log messages | match link' to correlate timestamps. In production, a single faulty SFP can cause routing protocol adjacencies (OSPF/BGP) to reset repeatedly, leading to network instability beyond just the local interface.
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.
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: Faulty SFP module — The log messages show repeated link up/down events (interface flapping). In Junos, interface flapping is most commonly caused by a faulty SFP module, as physical layer issues like degraded optics or loose connections trigger continuous link transitions. The logs typically show 'link up' followed by 'link down' without any protocol-level errors, pointing to a hardware fault rather than configuration mismatches.
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
Courseiva creates original exam-style practice questions with explanations and wrong-answer analysis. It does not publish real exam questions, exam dumps, or protected exam content. Learn why practice questions differ from exam dumps →
These questions test the same concept from different angles. Work through them to make sure you can recognise it however the exam phrases it.
Variation 1. Refer to the exhibit. Based on the log output, what is the most likely issue with interface ge-0/0/1?
hard
✓ A.Interface is experiencing flapping
B.Interface is up and stable
C.Interface is administratively down
D.Interface is configured incorrectly
Why A: The log output shows repeated 'link UP' and 'link DOWN' events for interface ge-0/0/1 within a short time window, which is the classic symptom of interface flapping. Flapping typically occurs due to physical layer issues such as faulty cables, damaged transceivers, or marginal signal integrity, causing the interface to continuously transition between up and down states.
Last reviewed: Jun 24, 2026
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