The answer is that the interface ge-0/0/0 is experiencing a physical layer issue causing it to flap. This is correct because the log messages show repeated, rapid transitions between down and up states on the interface, which is the classic signature of interface flapping. In Junos, such flapping is almost always rooted in a physical layer problem—like a faulty cable, damaged transceiver, or loose connection—rather than a configuration or routing protocol issue. On the JNCIA-Junos exam, this question tests your ability to troubleshoot interface flapping from logs, a key skill for identifying connectivity problems without jumping to conclusions about Layer 2 or Layer 3 causes. A common trap is to blame the BFD session or OSPF adjacency, but those are symptoms of the underlying physical instability, not the root cause. Remember the memory tip: “Flapping fingers point to physical fingers”—when you see rapid up/down cycles, always suspect the cable or optics first.
JNCIA-JUNOS Operational Monitoring and Maintenance Practice Question
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.
Exhibit
Refer to the exhibit.
% show log messages | last 10
Dec 10 10:45:12 router rpd[1234]: RPD_ISIS_ADJ_DOWN: ISIS adjacency down for interface ge-0/0/0.0, reason: hold timeout
Dec 10 10:45:12 router mib2d[5678]: SNMP_TRAP_LINK_DOWN: link down on interface ge-0/0/0.0, ifIndex 512
Dec 10 10:45:15 router rpd[1234]: RPD_ISIS_ADJ_UP: ISIS adjacency up for interface ge-0/0/0.0
Refer to the exhibit. A network administrator notices intermittent connectivity issues. Based on the log messages, 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.
Refer to the exhibit.
% show log messages | last 10
Dec 10 10:45:12 router rpd[1234]: RPD_ISIS_ADJ_DOWN: ISIS adjacency down for interface ge-0/0/0.0, reason: hold timeout
Dec 10 10:45:12 router mib2d[5678]: SNMP_TRAP_LINK_DOWN: link down on interface ge-0/0/0.0, ifIndex 512
Dec 10 10:45:15 router rpd[1234]: RPD_ISIS_ADJ_UP: ISIS adjacency up for interface ge-0/0/0.0
A
The ISIS protocol has a configuration error that prevents adjacency.
Why wrong: ISIS adjacency goes down due to hold timeout (likely caused by the link down), but it recovers quickly, indicating no configuration error.
B
The interface ge-0/0/0 is experiencing a physical layer issue causing it to flap.
The logs show a link down followed by a rapid recovery, consistent with a physical layer problem like a bad cable or SFP.
C
An SNMP trap is causing a loop in the network.
Why wrong: SNMP traps are notifications, not causes of loops. The trap is a result of the link down event.
D
The router's MIB is corrupted causing false logs.
Why wrong: There is no evidence of MIB corruption. The logs are consistent with real events.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The interface ge-0/0/0 is experiencing a physical layer issue causing it to flap.
The log shows rapid down/up events (flapping) on interface ge-0/0/0. This pattern typically indicates a physical layer problem such as a faulty cable or transceiver. Option B is plausible but the traps are symptoms, not the cause. Option C is less likely because the adjacency recovers quickly. Option D is unsupported by the logs.
Key principle: OSPF neighbour adjacency depends on matching area, hello/dead timers, network type, and authentication — IP reachability alone is not enough.
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 ISIS protocol has a configuration error that prevents adjacency.
Why it's wrong here
ISIS adjacency goes down due to hold timeout (likely caused by the link down), but it recovers quickly, indicating no configuration error.
✓
The interface ge-0/0/0 is experiencing a physical layer issue causing it to flap.
Why this is correct
The logs show a link down followed by a rapid recovery, consistent with a physical layer problem like a bad cable or SFP.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
SNMP traps are notifications, not causes of loops. The trap is a result of the link down event.
✗
The router's MIB is corrupted causing false logs.
Why it's wrong here
There is no evidence of MIB corruption. The logs are consistent with real events.
Common exam traps
Common exam trap: OSPF can fail even when IP connectivity looks correct
OSPF neighbour formation depends on matching areas, timers, network type, authentication and passive-interface behaviour. Do not choose an answer only because the devices can ping.
Detailed technical explanation
How to think about this question
OSPF questions usually test the details that control adjacency and route selection. Read the neighbour state, area, router ID and interface configuration before deciding what is wrong.
KKey Concepts to Remember
OSPF neighbours must agree on key parameters.
Router ID selection can affect neighbour relationships and LSDB output.
OSPF cost influences the preferred path.
A route can appear in OSPF information but not become the installed route.
TExam Day Tips
→Check area mismatch first when OSPF adjacency fails.
→Review passive interfaces when a network is advertised but no neighbour forms.
→Use show ip ospf neighbor and show ip route clues carefully.
Key takeaway
OSPF neighbour adjacency depends on matching area, hello/dead timers, network type, and authentication — IP reachability alone is not enough.
Real-world example
How this comes up in practice
A network engineer at a university connects two campus buildings via a fibre link. Both routers run OSPF, but no adjacency forms — even though both routers can ping each other. The engineer finds one router is in area 0 and the other in area 1. OSPF adjacency requires matching area numbers, hello/dead timers, and network type. IP reachability alone is not enough.
What to study next
Got this wrong? Here's your next step.
Review OSPF neighbour requirements — matching area type, hello and dead timers, network type, stub flags, and authentication. Study show ip ospf neighbor states (INIT, 2-WAY, FULL). Then practise related JNCIA-JUNOS OSPF questions on adjacency and route selection.
Operational Monitoring and Maintenance — This question tests Operational Monitoring and Maintenance — OSPF neighbours must agree on key parameters..
What is the correct answer to this question?
The correct answer is: The interface ge-0/0/0 is experiencing a physical layer issue causing it to flap. — The log shows rapid down/up events (flapping) on interface ge-0/0/0. This pattern typically indicates a physical layer problem such as a faulty cable or transceiver. Option B is plausible but the traps are symptoms, not the cause. Option C is less likely because the adjacency recovers quickly. Option D is unsupported by the logs.
What should I do if I get this JNCIA-JUNOS question wrong?
Review OSPF neighbour requirements — matching area type, hello and dead timers, network type, stub flags, and authentication. Study show ip ospf neighbor states (INIT, 2-WAY, FULL). Then practise related JNCIA-JUNOS OSPF questions on adjacency and route selection.
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?
OSPF neighbours must agree on key parameters.
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