Quick Answer
The answer is a faulty transceiver, as a high number of input errors with no output errors on a Juniper interface points directly to a Layer 1 issue at the receiving end. When diagnosing input errors due to a faulty transceiver, the transceiver’s signal degradation causes CRC, framing, or alignment errors that the router counts as input errors, while the absence of output errors confirms the router’s own transmission is clean. On the JNCIA-Junos exam, this scenario tests your ability to isolate physical-layer problems from higher-layer issues, and a common trap is to blame cable length or duplex mismatches—but those often produce both input and output errors. A reliable memory tip is “input errors, no output? Check the receiver’s optics,” as the transceiver is the first component the incoming signal hits.
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.
A router is experiencing intermittent packet loss. The engineer runs 'show interfaces ge-0/0/0 extensive' and notices a high number of input errors but no output errors. 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.
The cable is too long
Why wrong: Cable length affects signal but not specifically input errors.
A faulty transceiver
Faulty optics can cause input errors.
MTU mismatch
Why wrong: MTU mismatch causes giant frames, not typical input errors.
The interface is oversubscribed
Why wrong: Oversubscription causes output drops, not input errors.
Answer choices
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
A faulty transceiver
A high number of input errors with no output errors on a Juniper interface typically indicates a Layer 1 issue at the receiving end. A faulty transceiver can cause signal degradation, leading to CRC errors, framing errors, or alignment errors, all of which are counted as input errors. Since output errors are absent, the problem is not with the router's transmission but with the incoming signal quality.
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.
The cable is too long
Why it's wrong here
Cable length affects signal but not specifically input errors.
A faulty transceiver
Why this is correct
Faulty optics can cause input errors.
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 giant frames, not typical input errors.
The interface is oversubscribed
Why it's wrong here
Oversubscription causes output drops, not input errors.
Common exam traps
The trap here is that candidates often confuse input errors with output drops or oversubscription, but input errors are strictly Layer 1 physical-layer issues, not congestion or configuration mismatches.
Trap categories for this question
Command / output trap
Oversubscription causes output drops, not input errors.
Detailed technical explanation
Input errors on a Juniper interface include CRC errors, framing errors, and alignment errors, which are often caused by faulty optics, dirty fiber, or electromagnetic interference. The 'show interfaces ge-0/0/0 extensive' command provides detailed error counters and can show the specific error type (e.g., CRC errors), helping isolate the issue to the physical layer. In real-world scenarios, swapping the transceiver with a known-good one is the first troubleshooting step, as transceivers are a common failure point due to heat or manufacturing defects.
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 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
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
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FAQ
Operational Monitoring and Maintenance — This question tests Operational Monitoring and Maintenance — Read the scenario before looking for a memorised answer..
The correct answer is: A faulty transceiver — A high number of input errors with no output errors on a Juniper interface typically indicates a Layer 1 issue at the receiving end. A faulty transceiver can cause signal degradation, leading to CRC errors, framing errors, or alignment errors, all of which are counted as input errors. Since output errors are absent, the problem is not with the router's transmission but with the incoming signal quality.
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 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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