Question 1easymultiple choice
Read the full MPLS explanation →Question 368 of 500
Quick Answer
The answer is to change the encapsulation on the PE subinterfaces to dot1q second-dot1q and enable preservation of the outer VLAN tag at the ingress PE. This resolves the issue because EoMPLS Martini Q-in-Q transport, as defined in RFC 4448, requires the ingress PE to treat the outer 802.1Q tag as part of the Ethernet payload rather than stripping it. The default dot1q encapsulation removes the outer tag, which breaks double-tagged frames; using dot1q second-dot1q explicitly preserves both tags, allowing transparent Q-in-Q transport across the pseudowire. On the Cisco SPCOR 350-501 exam, this scenario tests your understanding of Layer 2 VPN attachment circuit configurations, specifically how Martini encapsulation interacts with VLAN stacking. A common trap is assuming standard dot1q subinterfaces suffice for Q-in-Q, but they only handle single-tagged frames. Remember the memory tip: "second-dot1q saves the second tag" — if the customer needs double tags, you must configure second-dot1q to keep the outer tag intact.
350-501 Services Practice Question
This 350-501 practice question tests your understanding of services. 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 a point-to-point Layer 2 VPN across an MPLS network using Ethernet over MPLS (EoMPLS) with Martini encapsulation. The customer requires transparent transport of VLAN tags (Q-in-Q) between two sites. The provider configures the attachment circuits on the PE routers as VLAN subinterfaces with dot1q encapsulation. After configuration, the customer reports that only untagged frames pass through the pseudowire; double-tagged frames are dropped at the egress PE. Which action resolves the issue?
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
✓
Change the encapsulation on the PE subinterfaces to dot1q second-dot1q and enable the preservation of the outer VLAN tag at the ingress PE
Option D is correct because EoMPLS with Martini encapsulation (RFC 4448) supports Q-in-Q transparent transport only when the ingress PE is configured to preserve the outer VLAN tag. Using 'dot1q second-dot1q' encapsulation on the subinterface tells the PE to treat the outer tag as part of the payload and not strip it, allowing both tags to be carried across the pseudowire. Without this, the default dot1q encapsulation strips the outer tag, causing double-tagged frames to be dropped at the egress PE.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the distinction between 'dot1q' and 'dot1q second-dot1q' encapsulation on subinterfaces, where candidates mistakenly think that standard dot1q encapsulation will preserve double tags, but it actually strips the outer tag before encapsulation into the pseudowire.
Detailed technical explanation
How to think about this question
In EoMPLS (RFC 4448), the control word and encapsulation details are negotiated via LDP. When using 'dot1q second-dot1q' on a subinterface, the PE treats the outer 802.1Q tag as a service delimiter and leaves it intact, while the inner tag is carried as payload. This is critical for service providers offering transparent LAN services (TLS) where customer Q-in-Q frames must traverse the MPLS core unchanged. A common real-world scenario is a customer running a stacked VLAN (e.g., 802.1ad) across a provider MPLS network, where misconfiguration leads to dropped frames and troubleshooting requires checking the encapsulation type on the PE subinterface.
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
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Services — study guide chapter
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FAQ
Questions learners often ask
What does this 350-501 question test?
Services — This question tests Services — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Change the encapsulation on the PE subinterfaces to dot1q second-dot1q and enable the preservation of the outer VLAN tag at the ingress PE — Option D is correct because EoMPLS with Martini encapsulation (RFC 4448) supports Q-in-Q transparent transport only when the ingress PE is configured to preserve the outer VLAN tag. Using 'dot1q second-dot1q' encapsulation on the subinterface tells the PE to treat the outer tag as part of the payload and not strip it, allowing both tags to be carried across the pseudowire. Without this, the default dot1q encapsulation strips the outer tag, causing double-tagged frames to be dropped at the egress PE.
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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Last reviewed: Jun 11, 2026
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