The answer is that the IRB interfaces are not assigned to the same routing instance. On a Juniper EX switch, inter-VLAN routing relies on IRB (Integrated Routing and Bridging) interfaces acting as Layer 3 gateways for each VLAN. For traffic to flow between VLAN 100 and VLAN 200, both IRB interfaces must belong to a common routing instance—typically the default instance inet.0—so the switch can perform route lookup and forward packets between them. If each IRB is placed in a separate routing instance, no inter-VLAN route exists, breaking Layer 3 connectivity. On the JNCIA-Junos exam, this concept tests your understanding of how Junos separates Layer 2 bridging from Layer 3 routing, and a common trap is assuming that simply configuring IRB interfaces is enough without verifying they share a routing instance. A helpful memory tip: think of the routing instance as the neighborhood—if two houses (VLANs) are on different streets (routing instances), they can’t visit each other without a connecting road.
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 engineer is troubleshooting connectivity between two VLANs on the same Juniper EX switch. Hosts in VLAN 100 cannot ping hosts in VLAN 200. The switch has an IRB interface configured for each VLAN. Which configuration is most likely missing?
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.
Spanning Tree Protocol is not enabled on the switch
Why wrong: STP prevents loops but does not affect Layer 3 routing between VLANs.
B
The switchport mode is set to access instead of trunk
Why wrong: Access ports are for single VLANs; trunk ports carry multiple VLANs but do not affect Layer 3 routing.
C
The IRB interfaces are not assigned to the same routing instance
IRB interfaces must be in the same routing instance with routing between them enabled; default instance works, but if they are in different instances, routing fails.
D
DHCP relay is not configured on the IRB interfaces
Why wrong: DHCP relay is for obtaining IP addresses, not for routing between directly connected subnets.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The IRB interfaces are not assigned to the same routing instance
For hosts in different VLANs to communicate through an EX switch, the IRB interfaces must belong to the same routing instance to enable inter-VLAN routing. By default, each IRB is placed in the default routing instance (inet.0), but if they are assigned to separate routing instances, no route exists between them, breaking Layer 3 forwarding. Option C directly addresses 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.
✗
Spanning Tree Protocol is not enabled on the switch
Why it's wrong here
STP prevents loops but does not affect Layer 3 routing between VLANs.
✗
The switchport mode is set to access instead of trunk
Why it's wrong here
Access ports are for single VLANs; trunk ports carry multiple VLANs but do not affect Layer 3 routing.
✓
The IRB interfaces are not assigned to the same routing instance
Why this is correct
IRB interfaces must be in the same routing instance with routing between them enabled; default instance works, but if they are in different instances, routing fails.
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.
✗
DHCP relay is not configured on the IRB interfaces
Why it's wrong here
DHCP relay is for obtaining IP addresses, not for routing between directly connected subnets.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates confuse Layer 2 issues (like trunking or STP) with Layer 3 routing, assuming VLAN-to-VLAN ping failures must be caused by a missing trunk or STP misconfiguration, when the actual missing piece is the routing instance assignment for the IRB interfaces.
Detailed technical explanation
How to think about this question
On Junos, IRB interfaces act as the gateway for each VLAN and must be placed in the same routing instance (e.g., default) to allow routing between them. If IRBs are in separate routing instances (e.g., VLAN-100 and VLAN-200), they are isolated at Layer 3, and no inter-VLAN route exists unless explicit routing policies or leak routes are configured. This is a common misconfiguration when using virtual-router or non-default routing instances without proper inter-instance routing.
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.
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 IRB interfaces are not assigned to the same routing instance — For hosts in different VLANs to communicate through an EX switch, the IRB interfaces must belong to the same routing instance to enable inter-VLAN routing. By default, each IRB is placed in the default routing instance (inet.0), but if they are assigned to separate routing instances, no route exists between them, breaking Layer 3 forwarding. Option C directly addresses 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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