JNCIA-JUNOS Networking Fundamentals Practice Question
This JNCIA-JUNOS practice question tests your understanding of networking fundamentals. This is a configuration task: choose the command set that satisfies every stated requirement. Small differences — like 'secret' vs 'password' or 'transport input ssh' vs 'all' — change whether the answer is correct. 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 Juniper EX switch has two VLANs configured, each with an IRB interface assigned an IP address. Pings between hosts in different VLANs fail. 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
✓
IP routing is not enabled globally: set routing-options router-id 10.0.0.1; set routing-options rib inet.0 static route 0.0.0.0/0 next-hop 203.0.113.1
Option B is correct because on a Juniper EX switch, inter-VLAN routing requires IP routing to be explicitly enabled. By default, routing is disabled on Juniper switches, so even with IRB interfaces configured, traffic between VLANs will not be forwarded unless routing is enabled globally using the `set routing-options router-id` command (which activates the routing engine) and a default route is configured to provide a next-hop for inter-VLAN traffic. Without this, the switch will not perform Layer 3 forwarding between the IRB interfaces.
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 VLANs are not configured with the same VLAN ID
Why it's wrong here
VLANIDs are unique within a switch, but they do not affect routing.
✓
IP routing is not enabled globally: set routing-options router-id 10.0.0.1; set routing-options rib inet.0 static route 0.0.0.0/0 next-hop 203.0.113.1
Why this is correct
Inter-VLAN routing requires IP routing to be enabled. The switch forwards between IRBs only if routing is on.
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 needs an external router configured for VLAN routing
Why it's wrong here
The switch can route between VLANs internally if properly configured.
✗
The IRB interfaces are not in the same routing instance
Why it's wrong here
By default, all IRBs are in the same default routing instance.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates assume IRB interfaces automatically enable inter-VLAN routing, similar to Cisco SVIs, but Juniper requires explicit routing configuration to activate Layer 3 forwarding.
Detailed technical explanation
How to think about this question
On Juniper EX switches, IRB interfaces are Layer 3 VLAN interfaces that require the routing engine to be active for packet forwarding between subnets. The `set routing-options router-id` command activates the routing engine, and a default route (or specific routes) provides the next-hop for inter-VLAN traffic. Without these, the switch operates purely as a Layer 2 device, even with IRB interfaces configured, which is a common point of confusion for those familiar with Cisco switches where IP routing is enabled by default on SVIs.
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: IP routing is not enabled globally: set routing-options router-id 10.0.0.1; set routing-options rib inet.0 static route 0.0.0.0/0 next-hop 203.0.113.1 — Option B is correct because on a Juniper EX switch, inter-VLAN routing requires IP routing to be explicitly enabled. By default, routing is disabled on Juniper switches, so even with IRB interfaces configured, traffic between VLANs will not be forwarded unless routing is enabled globally using the `set routing-options router-id` command (which activates the routing engine) and a default route is configured to provide a next-hop for inter-VLAN traffic. Without this, the switch will not perform Layer 3 forwarding between the IRB interfaces.
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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