350-501 Networking Practice Question
A network engineer is troubleshooting an MPLS L3VPN where the CE router is receiving the correct VPN prefixes from the PE, but traffic from the CE to those prefixes is being dropped. The PE has a default route pointing to the CE. What is the most likely cause?
⚠ Common exam trap
Cisco often tests the misconception that a VRF route alone is sufficient for forwarding, but in reality, the PE must have a global route to the BGP next-hop (typically the remote PE's loopback) for the MPLS label-switched path to function.
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 PE does not have a specific route for the destination in its global routing table.
The CE is receiving the correct VPN prefixes from the PE, so the VRF import/export is working. However, when the CE sends traffic to those prefixes, the PE must forward the packets. The PE has a default route pointing to the CE, but if the PE's global routing table lacks a specific route for the destination prefix (which is normal for L3VPN, as VPN routes are in the VRF, not the global table), the PE will drop the traffic because it cannot find a valid next hop in the global table for the outer IP header. This is a classic issue where the PE's global table must have a route to the CE's loopback or the PE-CE link subnet to enable recursive forwarding.
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 on the PE is not configured with the correct route-target import.
Why it's wrong here
The CE receives the correct prefixes, so route-target import is working.
- ✓
The PE does not have a specific route for the destination in its global routing table.
Why this is correct
Without a specific route, the PE may not push the correct MPLS label, causing the core to drop the packet.
- ✗
The CE does not have a route back to the PE's loopback.
Why it's wrong here
Return traffic uses the PE's loopback as next hop, but the CE has a default route, so this is not an issue.
- ✗
The PE-CE link MTU is smaller than the packet size.
Why it's wrong here
This would cause fragmentation issues, not drops for all traffic.
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JA
Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This 350-501 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 350-501 exam.