JN0-106 Networking Fundamentals Practice Question
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?
⚠ Common exam trap
Test-takers frequently 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.
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
✓
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.
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 scenario explicitly states that the IRB interface has an IP address assigned, so this cannot be the root cause. For inter-VLAN routing on Junos, each IRB unit needs an IP address in the corresponding VLAN's subnet to act as the gateway for that VLAN. If the IRB lacked an IP, hosts would have no valid gateway and traffic would never leave the VLAN, which is not the reported symptom. Since the stem confirms an address is present, this option is incorrect.
- ✗
The VLANs are on different switches.
Why it's wrong here
Inter-VLAN routing does not require the VLANs to reside on separate switches; a single switch with IRB interfaces can route between VLANs locally. Even if the VLANs extend across multiple switches, you would typically configure trunk ports and ensure the IRB exists on one switch or use an external router, but this topology alone does not break inter-VLAN routing. The symptom described—traffic failing between two VLANs—would occur regardless of the underlying switch placement if the VLAN definitions are missing. Thus, this option is not the cause.
- ✓
The VLANs are not defined on the switch.
Why this is correct
For inter-VLAN routing to work in Junos, each VLAN must first be defined under the [edit vlans] hierarchy so the switch knows which VLAN ID or name to associate with the ports and IRB interface. Without that definition, the IRB interface cannot be logically bound to a broadcast domain, and traffic cannot be forwarded between the two VLAN subnets. The stem confirms the IRB has an IP address, but if the VLANs themselves are not configured, the switch has no way to match incoming frames to a routing instance. This missing VLAN definition is exactly what breaks the traffic flow, making this the correct answer.
- ✗
The switch ports are not configured for VLAN tagging.
Why it's wrong here
VLAN tagging is only necessary on trunk ports that carry multiple VLANs over a single link, not on access ports that belong to a specific VLAN. In a single-switch inter-VLAN routing scenario, the IRB interface terminates each VLAN's subnet, and the access ports forward untagged frames into their assigned VLAN. Even if you had trunk ports, the lack of tagging alone would not prevent the IRB from routing between VLANs; it would just affect how frames arrive at the switch. Therefore, this is not the cause of the reported traffic failure.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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