The answer is ge-0/0/1, because its output shows 'Link: DOWN', which directly indicates a physical layer failure that prevents the interface from passing traffic. In Junos, a down link means the interface cannot forward any packets, regardless of higher-level protocol configurations like IP addresses or routing protocols—this is the most fundamental operational issue to identify when troubleshooting connectivity. On the JNCIA-Junos exam, this concept tests your ability to read interface status output and distinguish between physical (link) issues and logical (protocol) issues; a common trap is assuming a configured IP address means the interface is operational. Remember the memory tip: "Link down, traffic drown"—if the link is down, no traffic flows, no matter what else is configured.
JNCIA-JUNOS User Interfaces Practice Question
This JNCIA-JUNOS practice question tests your understanding of user interfaces. 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 interfaces terse
Interface Admin Link Proto Local Remote
ge-0/0/0 up up inet 10.1.1.1/24
ge-0/0/1 up down inet 10.1.2.1/24
ge-0/0/2 down down
lo0 up up inet 192.168.1.1/32
inet6 fe80::1/128
Refer to the exhibit. Which interface has an operational issue that prevents it from passing traffic?
Refer to the exhibit.
user@router> show interfaces terse
Interface Admin Link Proto Local Remote
ge-0/0/0 up up inet 10.1.1.1/24
ge-0/0/1 up down inet 10.1.2.1/24
ge-0/0/2 down down
lo0 up up inet 192.168.1.1/32
inet6 fe80::1/128
A
ge-0/0/1
The link is down while administratively up, indicating a physical issue.
B
lo0
Why wrong: The loopback interface is up/up and operational.
C
ge-0/0/2
Why wrong: This interface is administratively down, which could be intentional.
D
ge-0/0/0
Why wrong: This interface is up/up and operational.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
ge-0/0/1
Interface ge-0/0/1 shows 'Link: DOWN' in the output, indicating a physical layer issue such as a disconnected cable, faulty transceiver, or administrative shutdown. A down link prevents any traffic from being forwarded over that interface, regardless of its protocol configuration.
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.
✓
ge-0/0/1
Why this is correct
The link is down while administratively up, indicating a physical issue.
Related concept
Read the scenario before looking for a memorised answer.
✗
lo0
Why it's wrong here
The loopback interface is up/up and operational.
✗
ge-0/0/2
Why it's wrong here
This interface is administratively down, which could be intentional.
✗
ge-0/0/0
Why it's wrong here
This interface is up/up and operational.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates may focus on protocol state (e.g., 'Protocol: DOWN') or assume a loopback interface can pass transit traffic, but the key operational issue is the physical link state being down, which immediately blocks all traffic on that interface.
Detailed technical explanation
How to think about this question
In Junos, the 'show interfaces terse' command displays the administrative status (Admin) and link status (Link) for each interface. A 'Link: DOWN' state can result from a missing cable, incorrect speed/duplex mismatch, or the interface being disabled with 'disable' at the [edit interfaces ge-0/0/1] hierarchy. The physical layer must be 'Up' before any Layer 2 or Layer 3 protocols can function, as the link state directly controls the interface's ability to transmit frames.
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 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.
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.
User Interfaces — This question tests User Interfaces — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: ge-0/0/1 — Interface ge-0/0/1 shows 'Link: DOWN' in the output, indicating a physical layer issue such as a disconnected cable, faulty transceiver, or administrative shutdown. A down link prevents any traffic from being forwarded over that interface, regardless of its protocol configuration.
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.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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