The answer is a failed MPLS LDP session between the 6PE routers or between the 6PE and P routers. In a 6PE label switched path, the BGP next-hop for an IPv6 prefix is the IPv4 address of the remote 6PE router, and the local 6PE router must have a working LDP session to distribute a label for that IPv4 next-hop. Without that label, the routing table marks the next-hop as unreachable, even though the BGP table contains the prefix. This scenario is a classic trap on the Cisco CCNP ENARSI 300-410 exam: engineers often focus on MP-BGP or the tunnel being up, but forget that 6PE relies on an underlying IPv4 MPLS LSP. The key insight is that 6PE uses IPv4 transport labels, not IPv6 labels, so LDP must be functional for the BGP next-hop to be reachable. Memory tip: “6PE needs LDP for the IPv4 hop—no label, no path.”
300-410 IPv6 Tunneling Techniques Practice Question
This 300-410 practice question tests your understanding of ipv6 tunneling techniques. 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 IPv6 connectivity problem across an IPv4 MPLS network using 6PE. The 6PE routers have MP-BGP sessions to exchange IPv6 prefixes, and the tunnel between them is up. However, a customer edge router behind one 6PE router cannot reach an IPv6 prefix behind the other 6PE router. The engineer checks the 6PE router's BGP table and sees the prefix, but the routing table shows the next-hop as unreachable. 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 MPLS LDP session between the 6PE routers or between the 6PE and P routers is down, so no label exists for the BGP next-hop.
In 6PE, the BGP next-hop for an IPv6 prefix is the IPv4 address of the remote 6PE router. The 6PE router must have a label-switched path (LSP) to that IPv4 next-hop, which requires a working MPLS LDP session to distribute a label for that IPv4 address. If the LDP session is down, no label exists for the BGP next-hop, making it unreachable in the routing table even though the BGP table contains the prefix.
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 MPLS LDP session between the 6PE routers or between the 6PE and P routers is down, so no label exists for the BGP next-hop.
Why this is correct
Correct because 6PE relies on MPLS labels to reach the remote 6PE router; without a label, the next-hop is unreachable, and traffic is dropped.
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 6PE router is missing the 'ipv6 unicast-routing' command.
Why it's wrong here
Incorrect because the router has BGP routes, so IPv6 unicast routing is enabled.
✗
The tunnel interface is not in the VRF of the customer.
Why it's wrong here
Incorrect because 6PE does not use VRFs; it uses global IPv6 routing with MPLS labels.
✗
The remote 6PE router is not advertising the IPv6 prefix via BGP.
Why it's wrong here
Incorrect because the BGP table has the prefix, so it is advertised.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the distinction between BGP table presence and routing table reachability, trapping candidates who assume that seeing the prefix in BGP guarantees it is usable for forwarding.
Detailed technical explanation
How to think about this question
6PE relies on MPLS LDP to resolve the BGP next-hop (the IPv4 address of the remote 6PE router) into a label. The IPv6 prefix is encapsulated with two labels: the inner label (from BGP) and the outer label (from LDP for the next-hop). If LDP is down, the outer label is missing, and the router cannot forward the packet, marking the next-hop as unreachable in the IPv6 routing table. This is a common issue when MPLS core is misconfigured or LDP neighbors are not established.
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 small business has 20 workstations on the 192.168.1.0/24 network and one public IP from its ISP. The router uses PAT (NAT overload) so all 20 devices share one public address using different source ports. NAT questions test whether you understand the four address terms and which direction each translation applies.
Related glossary terms
Concepts from this question explained
These glossary pages explain the core terms tested in this 300-410 question in full detail.
IPv6 Tunneling Techniques — This question tests IPv6 Tunneling Techniques — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The MPLS LDP session between the 6PE routers or between the 6PE and P routers is down, so no label exists for the BGP next-hop. — In 6PE, the BGP next-hop for an IPv6 prefix is the IPv4 address of the remote 6PE router. The 6PE router must have a label-switched path (LSP) to that IPv4 next-hop, which requires a working MPLS LDP session to distribute a label for that IPv4 address. If the LDP session is down, no label exists for the BGP next-hop, making it unreachable in the routing table even though the BGP table contains the prefix.
What should I do if I get this 300-410 question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
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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