JN0-106 Operational Monitoring and Maintenance Practice Question
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?
⚠ Common exam trap
A common mix-up: candidates 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.
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.
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
Clearing the MAC address table targets Layer 2 forwarding state, not the physical layer. MAC flapping occurs when a host moves between ports or a loop exists, causing the router to repeatedly relearn addresses; it would be visible as MAC move notifications, not as CRC or frame errors. Clearing the table would temporarily disrupt forwarding while leaving the underlying cable or connector defect untouched, so this action neither diagnoses nor resolves the observed error counters.
- ✗
Administer a 'shutdown' and 'no shutdown' on the interface to reset the line protocol.
Why it's wrong here
Issuing 'shutdown' and 'no shutdown' reinitializes the interface and may reset the local error counters, but it does not repair the physical medium. The line protocol is likely already up; the presence of CRC and frame errors indicates that frames are arriving corrupted due to signal integrity problems. Once the interface comes back up, the same bad cable or connector will produce fresh CRC errors, so this is at best a temporary counter reset and at worst a misleading false positive that delays the real fix.
- ✓
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 and frame errors indicate that frames are receiving an invalid FCS, which is a classic symptom of a physical-layer issue such as a damaged cable, loose connector, or excessive signal attenuation. The problem appeared after a recent cable change, which strongly implicates the new cable or its seating. Replacing that cable with a known-good one is the correct first step because it directly tests the most likely failure point, and a clean interface afterward confirms the diagnosis.
- ✗
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 and duplex mismatches typically produce late collisions, runts, and alignment errors on a half-duplex side, not a preponderance of CRC/frame errors on a point-to-point link. Modern Junos interfaces with auto-negotiation will display the negotiated parameters, and if the router and CPE both support the same speed, a mismatch is unlikely unless manually configured incorrectly. Since the reported errors are FCS-related and appeared after a cable swap, they point to the physical medium rather than negotiation parameters, so changing autonegotiation settings would not reduce the CRC count.
Visual reference
Quick reference
OSI Model Reference
| Layer | Name | PDU | Key Protocols / Devices |
|---|---|---|---|
| 7 | Application | Data | HTTP, HTTPS, DNS, SMTP, FTP, SSH |
| 6 | Presentation | Data | TLS / SSL, JPEG, ASCII encoding |
| 5 | Session | Data | NetBIOS, RPC, SIP |
| 4 | Transport | Segment / Datagram | TCP, UDP |
| 3 | Network | Packet | IP, ICMP, OSPF — Routers |
| 2 | Data Link | Frame | Ethernet, Wi-Fi, PPP — Switches, Bridges |
| 1 | Physical | Bits | Cables, NICs, Hubs, Repeaters |
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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.