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.
A network administrator is configuring a new interface and wants to ensure that the interface is enabled and can pass traffic. Which configuration element is required?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
set interfaces ge-0/0/0 unit 0 family inet address 192.168.1.1/24
Option B is correct because in Junos, an interface is administratively enabled by default (no explicit 'enable' command is needed), but to pass traffic it requires a logical unit with a configured protocol family and an IP address. The command 'set interfaces ge-0/0/0 unit 0 family inet address 192.168.1.1/24' creates unit 0, assigns the IPv4 address, and implicitly enables the interface for traffic forwarding. Without an address under a family, the interface cannot pass IP traffic even if it is administratively up.
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.
✗
set interfaces ge-0/0/0 enable
Why it's wrong here
The 'enable' keyword is not a valid configuration statement; interfaces are enabled by default unless 'disable' is configured.
✓
set interfaces ge-0/0/0 unit 0 family inet address 192.168.1.1/24
Why this is correct
Configuring an IP address on the interface implicitly enables it.
Related concept
Read the scenario before looking for a memorised answer.
✗
set interfaces ge-0/0/0 unit 0 family inet
Why it's wrong here
Missing the address statement; the interface is still not fully configured to pass traffic.
✗
set interfaces ge-0/0/0 disable
Why it's wrong here
This command disables the interface.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates familiar with Cisco IOS may expect an explicit 'no shutdown' command (or an 'enable' keyword) to bring an interface up, but Junos interfaces are enabled by default, and the critical missing piece is the IP address under the logical unit, not an administrative enable command.
Trap categories for this question
Keyword trap
The 'enable' keyword is not a valid configuration statement; interfaces are enabled by default unless 'disable' is configured.
Command / output trap
This command disables the interface.
Detailed technical explanation
How to think about this question
In Junos, each physical interface can have multiple logical units (unit numbers), and each unit can run multiple protocol families (inet, inet6, mpls, etc.). The 'family inet address' statement not only assigns the IP address but also triggers the creation of a local route for that address in the routing table and enables ARP on that subnet. Without an address, the interface will show 'up' but will not respond to ARP requests or forward IP packets, making it effectively non-functional for IP traffic. This design allows a single physical interface to carry multiple Layer 3 networks or protocols simultaneously.
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.
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: set interfaces ge-0/0/0 unit 0 family inet address 192.168.1.1/24 — Option B is correct because in Junos, an interface is administratively enabled by default (no explicit 'enable' command is needed), but to pass traffic it requires a logical unit with a configured protocol family and an IP address. The command 'set interfaces ge-0/0/0 unit 0 family inet address 192.168.1.1/24' creates unit 0, assigns the IPv4 address, and implicitly enables the interface for traffic forwarding. Without an address under a family, the interface cannot pass IP traffic even if it is administratively up.
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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Question Discussion
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