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 10.0.0.1/24;
}
}
user@router> show interfaces terse ge-0/0/0
Interface Admin Link Proto Local Remote
ge-0/0/0 up up inet 10.0.0.1/24
user@router> monitor traffic interface ge-0/0/0 extensive
...
verbose output suppressed
...
PACKET: 0 received, 0 sent
Based on the exhibit, the interface appears to be up and has an IP address, but the 'monitor traffic' output shows no packets. 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.
Exhibit
Refer to the exhibit.
user@router> show configuration interfaces ge-0/0/0
unit 0 {
family inet {
address 10.0.0.1/24;
}
}
user@router> show interfaces terse ge-0/0/0
Interface Admin Link Proto Local Remote
ge-0/0/0 up up inet 10.0.0.1/24
user@router> monitor traffic interface ge-0/0/0 extensive
...
verbose output suppressed
...
PACKET: 0 received, 0 sent
A
The interface is not in promiscuous mode, and 'monitor traffic' requires that mode to capture packets.
By default, Junos captures only packets destined to the router; for all packets, interface must be in promiscuous mode.
B
The interface is administratively down.
Why wrong: The show interfaces terse output shows 'Admin' is 'up', so it is not administratively down.
C
The interface is an unnumbered Ethernet interface.
Why wrong: The interface has an IPv4 address configured, so it is not unnumbered.
D
A firewall filter is blocking all inbound traffic.
Why wrong: No firewall filter is shown in the configuration, and the interface is up.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The interface is not in promiscuous mode, and 'monitor traffic' requires that mode to capture packets.
Option A is correct because 'monitor traffic' captures packets by default only on the management interface or on interfaces configured in promiscuous mode. The exhibit shows an interface that is up with an IP address, but if it is not in promiscuous mode, 'monitor traffic' will not capture packets on that interface. Option B is incorrect because the interface is up, not administratively down. Option C is incorrect because an unnumbered interface still has an IP address derived from another interface, and the exhibit shows an IP address. Option D is incorrect because there is no indication of a firewall filter blocking traffic; the issue is with packet capture, not traffic flow.
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 interface is not in promiscuous mode, and 'monitor traffic' requires that mode to capture packets.
Why this is correct
By default, Junos captures only packets destined to the router; for all packets, interface must be in promiscuous mode.
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 show interfaces terse output shows 'Admin' is 'up', so it is not administratively down.
✗
The interface is an unnumbered Ethernet interface.
Why it's wrong here
The interface has an IPv4 address configured, so it is not unnumbered.
✗
A firewall filter is blocking all inbound traffic.
Why it's wrong here
No firewall filter is shown in the configuration, and the interface is up.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Many certification questions include familiar terms but test a specific constraint. Read the exact wording before choosing an answer that is generally true but wrong for this case.
Trap categories for this question
Command / output trap
The show interfaces terse output shows 'Admin' is 'up', so it is not administratively down.
Detailed technical explanation
How to think about this question
This question should be treated as a scenario, not a definition check. Identify the problem, the constraint and the best action. Then compare each option against those facts.
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.
Use explanations to understand the rule behind the answer.
TExam Day Tips
→Underline the problem statement mentally.
→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 practitioner preparing for the JNCIA-JUNOS exam encounters this exact type of scenario on the job. The correct answer here is not the most general option — it is the best answer for the specific constraint described. Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option. Real exam questions reward reading the full scenario before eliminating options, because the constraint defines which answer fits.
Visual reference
What to study next
Got this wrong? Here's your next step.
Identify which JNCIA-JUNOS exam domain this question belongs to, then review the specific concept being tested. Practise related questions in that domain and focus on understanding why each wrong answer is tempting — not just why the correct answer is right.
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 interface is not in promiscuous mode, and 'monitor traffic' requires that mode to capture packets. — Option A is correct because 'monitor traffic' captures packets by default only on the management interface or on interfaces configured in promiscuous mode. The exhibit shows an interface that is up with an IP address, but if it is not in promiscuous mode, 'monitor traffic' will not capture packets on that interface. Option B is incorrect because the interface is up, not administratively down. Option C is incorrect because an unnumbered interface still has an IP address derived from another interface, and the exhibit shows an IP address. Option D is incorrect because there is no indication of a firewall filter blocking traffic; the issue is with packet capture, not traffic flow.
What should I do if I get this JNCIA-JUNOS question wrong?
Identify which JNCIA-JUNOS exam domain this question belongs to, then review the specific concept being tested. Practise related questions in that domain and focus on understanding why each wrong answer is tempting — not just why the correct answer is right.
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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