- A
Spanning Tree Protocol enabled
Why wrong: STP prevents loops, does not affect Layer 3 connectivity.
- B
An IRB interface for the VLAN
An IRB interface provides Layer 3 functionality for a VLAN on MX or EX series.
- C
A Layer 3 switchport
Why wrong: Layer 3 switchports are for routed ports, not VLAN interfaces.
- D
A default gateway for the VLAN
Why wrong: Default gateway is for inter-VLAN communication, not intra-VLAN.
Question 270 of 514
Troubleshooting Missing IRB Interface in VLAN Configuration
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. A key principle to apply: iRB (Integrated Routing and Bridging) Interface. 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.
An engineer is troubleshooting a network issue where hosts on the same VLAN cannot communicate with each other. Which configuration element 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.
Quick Answer
The answer is a missing IRB interface for the VLAN. While hosts on the same VLAN communicate at Layer 2 using MAC addresses and do not require a Layer 3 gateway for intra-VLAN traffic, the IRB (Integrated Routing and Bridging) interface is essential when the VLAN needs to provide a default gateway for hosts that must reach other subnets or when inter-VLAN routing is required. On the JNCIA-Junos exam, this concept tests your understanding of how Junos bridges Layer 2 switching with Layer 3 routing; a common trap is assuming that a missing IRB interface only affects cross-VLAN traffic, but in scenarios where the VLAN itself is not properly instantiated as a routed interface, hosts may fail to communicate entirely if the switchports are not correctly assigned. Remember: the IRB interface acts as the VLAN’s Layer 3 gateway—without it, the VLAN cannot route, but for pure Layer 2 communication, always verify the VLAN membership on the access ports first. A helpful mnemonic is “IRB for Routing, VLAN for Bridging.”
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
An IRB interface for the VLAN
Hosts on the same VLAN communicate at Layer 2 and do not require any Layer 3 configuration such as an IRB interface or default gateway. The most likely missing configuration is that Spanning Tree Protocol (STP) is not enabled. Without STP, loops can cause broadcast storms and MAC address flapping, preventing communication between hosts in the same VLAN. Therefore, option A is correct.
Key principle: IRB (Integrated Routing and Bridging) Interface
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 enabled
Why it's wrong here
STP prevents loops, does not affect Layer 3 connectivity.
- ✓
An IRB interface for the VLAN
Why this is correct
An IRB interface provides Layer 3 functionality for a VLAN on MX or EX series.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
IRB (Integrated Routing and Bridging) Interface
- ✗
A Layer 3 switchport
Why it's wrong here
Layer 3 switchports are for routed ports, not VLAN interfaces.
- ✗
A default gateway for the VLAN
Why it's wrong here
Default gateway is for inter-VLAN communication, not intra-VLAN.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often confuse the need for a default gateway (Layer 3) with Layer 2 connectivity, assuming hosts on the same VLAN need a gateway to communicate, when in fact they communicate directly via ARP and MAC addresses.
Detailed technical explanation
How to think about this question
In Junos, an IRB interface acts as a Layer 3 gateway for a VLAN, allowing the VLAN to route traffic to other subnets. For intra-VLAN communication, the switch must have the VLAN defined and ports assigned to it; if the VLAN is not created or ports are in different VLANs, hosts cannot communicate. A common misconfiguration is forgetting to set the port mode to access and assign it to the correct VLAN, which isolates hosts even if they are on the same subnet.
KKey Concepts to Remember
- IRB (Integrated Routing and Bridging) Interface
- VLAN
- Layer 2 Communication
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
IRB (Integrated Routing and Bridging) Interface
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.
Visual reference
Quick reference
Access Control Model Comparison
| Model | Acronym | Who Controls Access? | Best For |
|---|---|---|---|
| Discretionary Access Control | DAC | Resource owner | Small teams, file shares |
| Mandatory Access Control | MAC | System / security labels | Classified govt / military |
| Role-Based Access Control | RBAC | Administrator (via roles) | Enterprise environments |
| Attribute-Based Access Control | ABAC | Policy engine (user + resource attributes) | Fine-grained, dynamic policies |
| Rule-Based Access Control | RuBAC | System rules / ACLs | Firewall rules, network ACLs |
What to study next
Got this wrong? Here's your next step.
Review iRB (Integrated Routing and Bridging) Interface, then practise related JNCIA-JUNOS questions on the same topic to reinforce the concept.
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Networking Fundamentals — study guide chapter
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FAQ
Questions learners often ask
What does this JNCIA-JUNOS question test?
Networking Fundamentals — This question tests Networking Fundamentals — IRB (Integrated Routing and Bridging) Interface.
What is the correct answer to this question?
The correct answer is: An IRB interface for the VLAN — Hosts on the same VLAN communicate at Layer 2 and do not require any Layer 3 configuration such as an IRB interface or default gateway. The most likely missing configuration is that Spanning Tree Protocol (STP) is not enabled. Without STP, loops can cause broadcast storms and MAC address flapping, preventing communication between hosts in the same VLAN. Therefore, option A is correct.
What should I do if I get this JNCIA-JUNOS question wrong?
Review iRB (Integrated Routing and Bridging) Interface, then practise related JNCIA-JUNOS questions on the same topic to reinforce the concept.
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?
IRB (Integrated Routing and Bridging) Interface
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Last reviewed: Jun 24, 2026
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