The answer is that the ping failure is most likely caused by the neighbor device not being configured or being unreachable. This is correct because while the interface shows a physical link is up and unit 0 is enabled, the absence of a neighbor discovery or ARP entry indicates that the local router cannot resolve a valid next-hop for the destination. Without a reachable neighbor, ICMP echo requests have no path to forward, resulting in a complete ping failure. On the Juniper Networks Certified Associate Junos JNCIA-Junos exam, this scenario tests your ability to distinguish between Layer 1 and Layer 2 issues—a common trap is assuming a green interface guarantees connectivity, but you must verify neighbor reachability in the ARP table or adjacency database. A useful memory tip is "Link up does not mean neighbor up," so always check for a resolved next-hop before troubleshooting higher layers.
JNCIA-JUNOS Junos OS Fundamentals Practice Question
This JNCIA-JUNOS practice question tests your understanding of junos os fundamentals. 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.
user@router> show configuration interfaces ge-0/0/0
unit 0 {
family inet {
address 10.0.0.1/30;
}
}
user@router> show interfaces terse ge-0/0/0
Interface Admin Link Proto Local Remote
ge-0/0/0 up up inet 10.0.0.1/30
user@router> ping 10.0.0.2 count 1
PING 10.0.0.2 (10.0.0.2): 56 data bytes
^C
--- 10.0.0.2 ping statistics ---
1 packets transmitted, 0 packets received, 100% packet loss
Based on the exhibit, what is the most likely reason for the ping failure?
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.
user@router> show configuration interfaces ge-0/0/0
unit 0 {
family inet {
address 10.0.0.1/30;
}
}
user@router> show interfaces terse ge-0/0/0
Interface Admin Link Proto Local Remote
ge-0/0/0 up up inet 10.0.0.1/30
user@router> ping 10.0.0.2 count 1
PING 10.0.0.2 (10.0.0.2): 56 data bytes
^C
--- 10.0.0.2 ping statistics ---
1 packets transmitted, 0 packets received, 100% packet loss
A
A firewall filter is blocking ICMP traffic.
Why wrong: No firewall filter configuration is shown.
B
The interface is disabled at the unit level.
Why wrong: No 'disable' statement is shown.
C
The neighbor device is not configured or is unreachable.
The ping fails, likely because the neighbor is not configured or there is no route to 10.0.0.2.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The neighbor device is not configured or is unreachable.
The ping failure is most likely due to the neighbor device not being configured or unreachable because the output shows the interface is up (Physical link is Up) and the unit is enabled (Unit 0 is enabled), but there is no neighbor discovery or ARP entry. Without a valid next-hop or neighbor reachability, ICMP echo requests cannot be forwarded, resulting in ping failure.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
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 neighbor device is not configured or is unreachable.
Why this is correct
The ping fails, likely because the neighbor is not configured or there is no route to 10.0.0.2.
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.
✗
The interface is administratively down.
Why it's wrong here
The exhibit shows 'up' for Admin.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often assume a ping failure is due to an interface being down or a firewall filter, but the exhibit clearly shows the interface is up and enabled, so the issue must be at the network layer with neighbor unreachability.
Trap categories for this question
Command / output trap
No firewall filter configuration is shown.
Detailed technical explanation
How to think about this question
In Junos, ping failure can occur even when the interface is up and IP is configured if the destination is not directly connected and no route exists, or if the neighbor is not responding to ARP. The 'ping' command uses ICMP echo requests, which require a valid route and ARP resolution to the next-hop; if the neighbor device is down or misconfigured, ARP fails and packets are dropped. This is distinct from interface-level issues, which would show 'down' or 'disabled' in the output.
KKey Concepts to Remember
Read the scenario before looking for a memorised answer.
Find the constraint that changes the correct option.
Eliminate answers that are true in general but not in this case.
TExam Day Tips
→Watch for words such as best, first, most likely and least administrative effort.
→Review why wrong options are wrong, not only why the correct option is correct.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
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.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Junos OS Fundamentals — This question tests Junos OS Fundamentals — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The neighbor device is not configured or is unreachable. — The ping failure is most likely due to the neighbor device not being configured or unreachable because the output shows the interface is up (Physical link is Up) and the unit is enabled (Unit 0 is enabled), but there is no neighbor discovery or ARP entry. Without a valid next-hop or neighbor reachability, ICMP echo requests cannot be forwarded, resulting in ping failure.
What should I do if I get this JNCIA-JUNOS question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
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?
Read the scenario before looking for a memorised answer.
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