hardMultiple Choice
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
6PE forwards IPv6 traffic by swapping the label bound to the IPv6 BGP next-hop. If LDP is down, no label exists for that next-hop, so the route appears in BGP but cannot be installed, leaving the next-hop unreachable.
- ✗
The 6PE router is missing the 'ipv6 unicast-routing' command.
Why it's wrong here
Enabling IPv6 unicast routing affects local IPv6 forwarding, not next-hop resolution for 6PE. The unreachable next-hop arises because the IPv4-mapped IPv6 next-hop address lacks a route in the global IPv4 table. This command would be correct when the router itself cannot forward any IPv6 traffic at all.
- ✗
The tunnel interface is not in the VRF of the customer.
Why it's wrong here
6PE tunnels are established in the global routing table, not within a customer VRF; placing the tunnel interface in a VRF would break the MP-BGP peering itself. VRF placement is correct for 6VPE, where per-customer IPv6 routing separation is required, not for the shared 6PE core.
- ✗
The remote 6PE router is not advertising the IPv6 prefix via BGP.
Why it's wrong here
The engineer already sees the prefix in the BGP table, so the remote router is advertising it; the unreachable next-hop points to a missing route to the IPv6 next-hop address. This option is tempting when a prefix is absent entirely, which is not the case here.
Go deeper
Related to this question
Learn chapter
Infrastructure Services: DHCP, NAT, and IPv6
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.