The answer is that LDP is not enabled on the core interfaces between the PEs, which directly causes the VC state to show as down. In an MPLS Layer 2 VPN using pseudowires, the VC status relies on a functioning MPLS label path between the provider edge routers; if LDP fails to exchange labels for the transport LSP, the pseudowire cannot be established, leaving the VC down regardless of correct xconnect configuration. On the ENCOR 350-401 exam, this question tests your understanding of the control plane dependency between LDP and VPWS, often appearing as a distractor where candidates mistakenly blame mismatched VC IDs, encapsulation types, or attachment circuit parameters—all of which would cause different failure symptoms. A common trap is forgetting that LDP must be enabled on every core interface in the path, not just on the loopback. Remember the mnemonic: “No LDP, no VC—labels must flow for the pseudowire to grow.”
350-401 MPLS Practice Question
This 350-401 practice question tests your understanding of mpls. 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 an MPLS L2VPN (VPWS) where two customer sites are connected via a pseudowire. The engineer has configured the xconnect on both PE routers, but the customer reports that the link is down. The 'show mpls l2transport vc' command on PE1 shows the VC state as 'down'. What is the most likely cause?
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
✓
LDP is not enabled on the core interfaces between the PEs.
In MPLS L2VPN, the VC state depends on the MPLS label path. If LDP is not exchanging labels for the pseudowire, the VC will remain down. Option A is correct. Option B is wrong because the VC ID must match; Option C is wrong because the VC type must match; Option D is wrong because the encapsulation must match.
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.
✓
LDP is not enabled on the core interfaces between the PEs.
Why this is correct
Correct because LDP is required to exchange labels for the pseudowire.
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 VC ID is different on the two PEs.
Why it's wrong here
Incorrect because the VC ID must match, but the question states it is configured, so this is not the issue.
✗
The VC type is not set to Ethernet.
Why it's wrong here
Incorrect because the VC type must match, but it is likely configured correctly.
✗
The encapsulation is set to VLAN instead of Ethernet.
Why it's wrong here
Incorrect because the encapsulation must match, but it is likely configured correctly.
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 350-401 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 350-401 questions on switching, trunking, and access-port configuration.
MPLS — This question tests MPLS — Access ports place end devices into a single VLAN..
What is the correct answer to this question?
The correct answer is: LDP is not enabled on the core interfaces between the PEs. — In MPLS L2VPN, the VC state depends on the MPLS label path. If LDP is not exchanging labels for the pseudowire, the VC will remain down. Option A is correct. Option B is wrong because the VC ID must match; Option C is wrong because the VC type must match; Option D is wrong because the encapsulation must match.
What should I do if I get this 350-401 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 350-401 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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