hardMultiple ChoiceObjective-mapped
300-410 Practice Question: Is troubleshooting an IPv6 connectivity problem…
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?
⚠ Common exam trap
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.
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
✓
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.
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.
- ✗
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.
Go deeper
Related to this question
Learn chapter
Introduction to ENARSI Exam and Network Fundamentals
Key term
MPLS Label Distribution
MPLS Label Distribution is the process by which routers exchange labels that tell them how to forward packets across a network without looking at the IP address each time.
Key term
LDP Protocol
LDP, or Label Distribution Protocol, is a protocol that routers use to automatically exchange labels that enable MPLS (Multiprotocol Label Switching) to create fast, efficient paths for data packets across a network.
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JA
Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This 300-410 practice question is part of Courseiva's free Cisco certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the 300-410 exam.