The answer is a VLAN mismatch on the trunk subinterfaces, specifically where Router1 uses encapsulation dot1q 10 while Router2 uses encapsulation dot1q 20. This is correct because VRF-Lite relies on 802.1Q trunking to separate traffic between VRFs; each subinterface must be tagged with the same VLAN ID on both ends of the link to form a single Layer 3 adjacency. When the VLAN IDs do not match, the routers cannot communicate for that VRF, even though the VRFs themselves are active and the physical trunk is up. On the Cisco CCNP ENARSI 300-410 exam, this scenario tests your understanding of VRF-Lite connectivity fundamentals and the common pitfall of misaligned encapsulation commands. A frequent trap is assuming that simply assigning a subinterface to a VRF is sufficient, but the VLAN tag must be identical on both sides. Memory tip: “Same tag, same VRF—mismatched tags break the link.”
300-410 VRF-Lite Practice Question
This 300-410 practice question tests your understanding of vrf-lite. 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 a VRF-Lite deployment where two routers are connected via a trunk link. Each router has two VRFs (VRF_A and VRF_B). The engineer configures subinterfaces on the trunk link, assigning each subinterface to a different VRF. However, traffic between the two routers for VRF_A is not working. The 'show vrf' command shows the VRFs are active. What is the most likely issue?
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
✓
The subinterface on Router1 is configured with 'encapsulation dot1q 10', but the subinterface on Router2 is configured with 'encapsulation dot1q 20'.
In VRF-Lite with trunk links, subinterfaces must be associated with the correct VRF and VLAN. A common mistake is not matching the VLAN IDs on the subinterfaces of both routers, or missing the 'encapsulation dot1q' command.
Key principle: A trunk being up does not mean the VLAN is allowed across it. Always verify the allowed VLAN list and whether the VLAN exists on both switches.
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 subinterface on Router1 is configured with 'encapsulation dot1q 10', but the subinterface on Router2 is configured with 'encapsulation dot1q 20'.
Why this is correct
Mismatched VLANIDs prevent the Layer 2 frames from being correctly tagged and forwarded between the VRFs.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Access ports place end devices into a single VLAN.
✗
The 'ip vrf forwarding VRF_A' command is missing on the main interface.
Why it's wrong here
VRF forwarding is applied on subinterfaces, not the main interface, for trunk links.
✗
The 'no ip routing' command is configured globally.
Why it's wrong here
Disabling IP routing would affect all traffic, not just VRF_A.
✗
The 'mtu' command is set differently on the two subinterfaces.
Why it's wrong here
MTU mismatch could cause packet drops, but it is less likely than a VLAN mismatch for complete traffic failure.
Common exam traps
Common exam trap: an active trunk can still block the VLAN you need
A trunk being up does not prove every VLAN is crossing it. Check allowed VLAN lists, native VLAN mismatch, VLAN existence and access-port assignment.
Detailed technical explanation
How to think about this question
VLAN questions usually combine access-port and trunking clues. The key is to identify whether the issue is local to one switchport, caused by the trunk, or caused by the VLAN not existing where it needs to exist.
KKey Concepts to Remember
Access ports place end devices into a single VLAN.
Trunk ports carry multiple VLANs between switches.
Allowed VLAN lists decide which VLANs can cross a trunk.
Native VLAN mismatch can create confusing symptoms.
TExam Day Tips
→Use show vlan brief to verify access VLANs.
→Use show interfaces trunk to verify trunk state and allowed VLANs.
→Do not treat every same-VLAN issue as a routing problem.
Key takeaway
A trunk being up does not mean the VLAN is allowed across it. Always verify the allowed VLAN list and whether the VLAN exists on both switches.
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.
Related glossary terms
Concepts from this question explained
These glossary pages explain the core terms tested in this 300-410 question in full detail.
Review VLAN allowed lists, native VLAN mismatch detection, and how to verify VLAN membership with show vlan brief and show interfaces trunk. Then practise related 300-410 questions on switching, trunking, and access-port configuration.
VRF-Lite — This question tests VRF-Lite — Access ports place end devices into a single VLAN..
What is the correct answer to this question?
The correct answer is: The subinterface on Router1 is configured with 'encapsulation dot1q 10', but the subinterface on Router2 is configured with 'encapsulation dot1q 20'. — In VRF-Lite with trunk links, subinterfaces must be associated with the correct VRF and VLAN. A common mistake is not matching the VLAN IDs on the subinterfaces of both routers, or missing the 'encapsulation dot1q' command.
What should I do if I get this 300-410 question wrong?
Review VLAN allowed lists, native VLAN mismatch detection, and how to verify VLAN membership with show vlan brief and show interfaces trunk. Then practise related 300-410 questions on switching, trunking, and access-port configuration.
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?
Access ports place end devices into a single VLAN.
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