Question 469 of 156
Interface Flapping: Faulty SFP Module as Cause
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?
Quick Answer
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.”
⚠ Common exam trap
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.
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
✓
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.
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.
- ✗
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.
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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Same concept, more angles
1 more way this is tested on JN0-106
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
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.
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