The answer is an MTU mismatch between the local and remote PE, along with an encapsulation type mismatch. These two conditions cause a VPLS pseudowire to show a 'down' status because the pseudowire control protocol, typically LDP or BGP, performs a capabilities negotiation during setup. If the encapsulation types—such as Ethernet versus VLAN—do not match, or if the MTU values differ between the endpoints, the negotiation fails and the pseudowire cannot transition to the 'up' state. On the Cisco SPCOR 350-501 exam, this concept tests your understanding of VPLS pseudowire signaling and the mandatory parameter consistency required for Layer 2 circuits. A common trap is assuming only MTU matters, but encapsulation mismatch is equally critical; both must be identical on the local and remote PE. Remember the mnemonic "E-MTU" — Encapsulation and MTU must match for the pseudowire to come up.
350-501 Networking Practice Question
This 350-501 practice question tests your understanding of networking. Read the scenario carefully and evaluate each option against the stated constraints before committing to an answer. 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 configuring VPLS on a Cisco ASR 9000. After verifying the pseudowire status, they notice that the 'pw status' shows 'down' for one of the PWs. Which two conditions could cause this? (Choose two.)
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Mismatched encapsulation type on the pseudowire (e.g., Ethernet vs. VLAN)
Option A is correct because a mismatched encapsulation type (e.g., Ethernet vs. VLAN) on the pseudowire causes the PW to fail to come up. In VPLS, the encapsulation must match between the local and remote PE for the pseudowire to be operational; otherwise, the PW status will show 'down' due to a negotiation failure.
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.
✓
Mismatched encapsulation type on the pseudowire (e.g., Ethernet vs. VLAN)
Why this is correct
Encapsulation must match between peers.
Related concept
Read the scenario before looking for a memorised answer.
Read the scenario before looking for a memorised answer.
✗
LDP session is missing
Why it's wrong here
LDP session is required for signaling; if missing, PW would not come up, but not directly causing 'down' status as described.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the distinction between conditions that cause a pseudowire to be 'down' versus conditions that affect VPLS forwarding but leave the PW 'up', leading candidates to incorrectly select VPLS ID mismatch (Option B) as a cause of PW failure.
Detailed technical explanation
How to think about this question
Under the hood, VPLS pseudowires use the control word and encapsulation type (RFC 4448) to signal the payload format. An MTU mismatch (Option D) causes the PW to go down because the LDP label mapping message includes the interface MTU, and if it differs, the PW is not activated. In real-world scenarios, MTU mismatches are common when one PE has jumbo frames enabled and the other does not, leading to a 'down' PW that can be verified with 'show l2vpn xconnect'.
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.
TExam Day Tips
→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 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
Got this wrong? Here's your next step.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Networking — This question tests Networking — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Mismatched encapsulation type on the pseudowire (e.g., Ethernet vs. VLAN) — Option A is correct because a mismatched encapsulation type (e.g., Ethernet vs. VLAN) on the pseudowire causes the PW to fail to come up. In VPLS, the encapsulation must match between the local and remote PE for the pseudowire to be operational; otherwise, the PW status will show 'down' due to a negotiation failure.
What should I do if I get this 350-501 question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
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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