The answer is a VC ID mismatch between the two PEs. In an MPLS L2VPN VPWS deployment, the VC ID acts as the unique identifier that binds the two ends of a pseudowire; if the IDs do not match, the pseudowire cannot establish a control connection, leaving the attachment circuit (AC) operationally up but unable to transport Layer 2 frames. This explains why the xconnect interface shows as up yet no MAC addresses are learned—the MPLS core LSP may be available, but the pseudowire itself remains down due to the mismatched VC label. On the Cisco SPCOR 350-501 exam, this scenario tests your understanding of pseudowire signaling and the importance of consistent VC IDs across both PEs; a common trap is to blame MTU or encapsulation errors, but those would typically produce interface errors or drops, not a clean lack of MAC learning. Remember the memory tip: “VC ID must be identical—if they differ, the pseudowire won’t deliver.”
350-501 MPLS and Segment Routing Practice Question
This 350-501 practice question tests your understanding of mpls and segment routing. 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 service provider is deploying MPLS L2VPN using Virtual Private Wire Service (VPWS). After configuration, the two CEs cannot ping each other. The PE routers show the xconnect interface is up, but no MAC addresses are learned. 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
✓
The VC ID mismatch between the two PEs
If the attachment circuit (AC) is up but no MAC learning occurs, the issue is likely that the MPLS core LSP is not available for the pseudowire. The pseudowire status may be down due to LSP issues. VC ID mismatch would cause the pseudowire to not come up. MTU or encapsulation issues would show errors.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
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 MTU on the core links is less than 1500
Why it's wrong here
MTU mismatch would cause fragmentation, but pseudowire would still be up.
✗
The control word is enabled on one PE but not the other
Why it's wrong here
Control word mismatch can cause issues, but typically the pseudowire would be up with errors.
✓
The VC ID mismatch between the two PEs
Why this is correct
VC ID must match for the pseudowire to be established.
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.
✗
The encapsulation type on the AC is different on the two PEs
Why it's wrong here
Encapsulation must match, but both are probably Ethernet, and the question implies it's up.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Many certification questions include familiar terms but test a specific constraint. Read the exact wording before choosing an answer that is generally true but wrong for this case.
Detailed technical explanation
How to think about this question
This question should be treated as a scenario, not a definition check. Identify the problem, the constraint and the best action. Then compare each option against those facts.
KKey Concepts to Remember
Read the scenario before looking for a memorised answer.
Find the constraint that changes the correct option.
Eliminate answers that are true in general but not in this case.
Use explanations to understand the rule behind the answer.
TExam Day Tips
→Underline the problem statement mentally.
→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
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
How this comes up in practice
A practitioner preparing for the 350-501 exam encounters this exact type of scenario on the job. The correct answer here is not the most general option — it is the best answer for the specific constraint described. Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option. Real exam questions reward reading the full scenario before eliminating options, because the constraint defines which answer fits.
What to study next
Got this wrong? Here's your next step.
Identify which 350-501 exam domain this question belongs to, then review the specific concept being tested. Practise related questions in that domain and focus on understanding why each wrong answer is tempting — not just why the correct answer is right.
MPLS and Segment Routing — This question tests MPLS and Segment Routing — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The VC ID mismatch between the two PEs — If the attachment circuit (AC) is up but no MAC learning occurs, the issue is likely that the MPLS core LSP is not available for the pseudowire. The pseudowire status may be down due to LSP issues. VC ID mismatch would cause the pseudowire to not come up. MTU or encapsulation issues would show errors.
What should I do if I get this 350-501 question wrong?
Identify which 350-501 exam domain this question belongs to, then review the specific concept being tested. Practise related questions in that domain and focus on understanding why each wrong answer is tempting — not just why the correct answer is right.
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?
Read the scenario before looking for a memorised answer.
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Question Discussion
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