hardMultiple Choice
300-410 Practice Question: An engineer is troubleshooting an MPLS L3VPN…
An engineer is troubleshooting an MPLS L3VPN where CE1 (10.1.1.0/24) cannot reach CE2 (10.2.2.0/24). The PE routers have MP-BGP peering and the VRF is configured with route-target import 100:100. On PE1, the show ip bgp vpnv4 vrf CUSTOMER command shows the route for 10.2.2.0/24 with a next-hop of 192.168.1.2 (the PE2 loopback), but the show ip route vrf CUSTOMER command does not have this route. The show mpls forwarding-table on PE1 does not show a label for 192.168.1.2. What is the most likely cause?
⚠ Common exam trap
The trap is assuming that MP-BGP alone is sufficient for L3VPN; candidates forget that an MPLS LSP (via LDP or RSVP-TE) is required for the transport label, and without it, VPN routes are not installed.
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
✓
LDP is not enabled on the core-facing interfaces of PE1 or the P routers.
The most likely cause is that LDP is not enabled on the core-facing interfaces of PE1 or the P routers. For an MPLS L3VPN, the transport label (outer label) is distributed via LDP. Without LDP, PE1 cannot learn a label for the next-hop 192.168.1.2 (PE2's loopback), so the BGP VPNv4 route cannot be installed in the VRF routing table because there is no valid label stack. The show mpls forwarding-table confirms the absence of a label for that next-hop.
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 VRF route-target import is missing on PE2.
Why it's wrong here
Import on PE2 governs routes PE2 receives, not the 10.2.2.0/24 prefix PE1 already holds in its VPNv4 table; the missing piece is the transport label to PE2's loopback. It tempts because import mismatches do drop routes, making this correct when remote prefixes never appear locally.
- ✓
LDP is not enabled on the core-facing interfaces of PE1 or the P routers.
Why this is correct
Without LDP on core-facing interfaces, no transport label exists for PE2's loopback, so the VPNv4 route's next hop is unresolvable and BGP cannot install it into the VRF table. The missing MPLS forwarding entry confirms this.
- ✗
The MP-BGP session is not using the loopback interface.
Why it's wrong here
The VPNv4 route already shows PE2's loopback as next-hop, proving the MP-BGP session is established and exchanging routes; the missing MPLS label for that loopback indicates no LSP exists. It tempts because peering over physical interfaces instead of loopbacks does break VPNv4, but here the session clearly works.
- ✗
The VRF on PE1 has the wrong route-target export.
Why it's wrong here
Export controls what PE1 advertises to remote PEs, yet the failing route is inbound from PE2 and already present in the VPNv4 table, so export cannot be the fault. It tempts because mismatched export route-targets do cause missing routes, making this correct when a local VRF's prefixes fail to reach remote PEs.
Go deeper
Related to this question
Learn chapter
BGP Route Selection and Policy
Key term
MP-BGP for VPN
MP-BGP for VPN is an extension of the Border Gateway Protocol that carries multiple types of network layer information to enable scalable, isolated virtual private networks over a shared provider infrastructure.
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.
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JA
Written and reviewed by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
Last reviewed September 2026 · checked against the official Cisco exam blueprint
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.