JN0-106 Operational Monitoring and Maintenance Practice Question
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?
⚠ Common exam trap
Watch out — 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.
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
✓
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.
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 cable is too long
Why it's wrong here
A cable run exceeding IEEE specifications (100m for twisted-pair) can introduce attenuation and crosstalk, but the immediate symptom is typically degraded signal-to-noise ratio, which may cause link flaps or excessive CRC errors — not the specific 'input errors' counter. Input errors on Junos interfaces are commonly tied to hardware or optics failures, not simple cable reach, and a length violation would likely cause sustained, not intermittent, issues.
- ✓
A faulty transceiver
Why this is correct
A faulty transceiver (SFP/SFP+/QSFP) is a classic source of receive-side errors: imperfect optical transmission or receiver degradation creates bit errors, FCS/CRC failures, and symbol errors that increment the input error counter. On Junos, an optics failure can appear intermittently as temperature or wavelength drifts, making it a prime suspect for sporadic packet loss with input errors. Swapping the optic is the standard first step to confirm.
- ✗
MTU mismatch
Why it's wrong here
MTU mismatch is a Layer 2/3 forwarding issue: when a frame arrives larger than the interface's configured MTU, it is discarded as a giant or fragmented, which appears in counters like 'giants' or as output drops, not as input errors. Input errors reflect physical-layer encoding or framing problems, whereas an MTU mismatch is a configuration mismatch that affects the transmit/receive path differently. Therefore, it would not cause the input error counter to increment in this scenario.
- ✗
The interface is oversubscribed
Why it's wrong here
Oversubscription occurs when aggregated ingress traffic exceeds an interface's egress capacity, causing packets to queue and eventually be dropped as output drops due to buffer exhaustion. This is a transmit-side congestion problem, entirely separate from the receive-side physical errors that the input error counter records. An oversubscribed interface would show output drops, not input errors, and would not produce the CRC or framing errors associated with this symptom.
Go deeper
Related to this question
About these practice questions
This JN0-106 question is part of Courseiva's 326-question bank — original exam-style content with full explanations and wrong-answer analysis, never real exam questions or exam dumps. Learn why practice questions differ from exam dumps →
JA
Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This JN0-106 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 JN0-106 exam.