JNCIA-JUNOS Networking Fundamentals Practice Question
This JNCIA-JUNOS practice question tests your understanding of networking 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 notices that traffic between two VLANs is not reaching its destination. The switch has an IRB interface configured with an IP address in each VLAN's subnet. What is the most likely missing configuration?
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.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The VLANs are not defined on the switch.
The IRB interface provides Layer 3 routing between VLANs, but it requires the VLANs themselves to be defined on the switch. If the VLANs are not defined, the switch cannot associate the IRB interface with the correct broadcast domains, and traffic will not be forwarded between them. Option C correctly identifies this missing 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.
✗
The IRB interface does not have an IP address configured.
Why it's wrong here
The stem states the IRB interface has an IP address, so this is not the issue.
✗
The VLANs are on different switches.
Why it's wrong here
Inter-VLAN routing can occur on a single switch; VLANs do not need to be on different switches.
✓
The VLANs are not defined on the switch.
Why this is correct
Without defining the VLANs, the switch cannot associate ports or IRB interfaces with them.
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 switch ports are not configured for VLAN tagging.
Why it's wrong here
VLAN tagging is used for trunk ports, not for inter-VLAN routing on a single switch.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often assume an IRB interface with an IP address is sufficient for inter-VLAN routing, overlooking the prerequisite that the VLAN must be defined and associated with the IRB in the switch configuration.
Detailed technical explanation
How to think about this question
In Junos, an IRB interface (also known as a routed VLAN interface or RVI) acts as the default gateway for each VLAN. The VLAN must be explicitly defined under the 'vlans' hierarchy with a VLAN ID, and the IRB interface must be bound to that VLAN using the 'vlan' statement under the IRB configuration. Without the VLAN definition, the IRB interface cannot participate in Layer 2 forwarding or ARP resolution for that subnet, causing traffic to fail even if the IP address is present.
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 help-desk technician troubleshoots why a newly connected PC cannot reach shared printers on the same floor. The cable is good, the switch port is active, but the PC is in VLAN 20 and the printers are in VLAN 10. The uplink trunk only allows VLAN 10. A trunk being up does not mean every VLAN crosses it.
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.
Networking Fundamentals — This question tests Networking Fundamentals — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The VLANs are not defined on the switch. — The IRB interface provides Layer 3 routing between VLANs, but it requires the VLANs themselves to be defined on the switch. If the VLANs are not defined, the switch cannot associate the IRB interface with the correct broadcast domains, and traffic will not be forwarded between them. Option C correctly identifies this missing 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.
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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Question Discussion
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