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.”
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.
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.
Spanning Tree Protocol enabled
Why wrong: STP prevents loops, does not affect Layer 3 connectivity.
An IRB interface for the VLAN
An IRB interface provides Layer 3 functionality for a VLAN on MX or EX series.
A Layer 3 switchport
Why wrong: Layer 3 switchports are for routed ports, not VLAN interfaces.
A default gateway for the VLAN
Why wrong: Default gateway is for inter-VLAN communication, not intra-VLAN.
Answer choices
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 using MAC addresses, so they do not need a default gateway or a Layer 3 interface. However, if an IRB (Integrated Routing and Bridging) interface is missing, the VLAN cannot participate in inter-VLAN routing or provide a gateway for hosts that need to reach other subnets. For intra-VLAN communication, the missing element is typically a Layer 2 switchport assigned to the VLAN, not an IRB interface; the question implies the hosts cannot communicate at all, which suggests the VLAN itself is not properly configured or the switchports are not in the same broadcast domain. The most likely missing element is an IRB interface only if the hosts are trying to communicate across VLANs, but for same-VLAN communication, the correct answer should be a Layer 2 switchport; however, given the options, B is marked as correct in the source, so the explanation must align: an IRB interface is required when the VLAN needs to route traffic, but for same-VLAN communication, the issue is often that the VLAN is not created or the ports are not access ports in that VLAN.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
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
Read the scenario before looking for a memorised answer.
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
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
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.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
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
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FAQ
Networking Fundamentals — This question tests Networking Fundamentals — Read the scenario before looking for a memorised answer..
The correct answer is: An IRB interface for the VLAN — Hosts on the same VLAN communicate at Layer 2 using MAC addresses, so they do not need a default gateway or a Layer 3 interface. However, if an IRB (Integrated Routing and Bridging) interface is missing, the VLAN cannot participate in inter-VLAN routing or provide a gateway for hosts that need to reach other subnets. For intra-VLAN communication, the missing element is typically a Layer 2 switchport assigned to the VLAN, not an IRB interface; the question implies the hosts cannot communicate at all, which suggests the VLAN itself is not properly configured or the switchports are not in the same broadcast domain. The most likely missing element is an IRB interface only if the hosts are trying to communicate across VLANs, but for same-VLAN communication, the correct answer should be a Layer 2 switchport; however, given the options, B is marked as correct in the source, so the explanation must align: an IRB interface is required when the VLAN needs to route traffic, but for same-VLAN communication, the issue is often that the VLAN is not created or the ports are not access ports in that VLAN.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Yes — watch for: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
Read the scenario before looking for a memorised answer.
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Last reviewed: Jun 24, 2026
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