The answer is that the PE lacks a specific route in its global routing table for the destination prefix. When the CE sends traffic toward the VPN prefixes it learned from the PE, the PE must forward the packets using its global table, not the VRF. Even though the PE has a default route pointing to the CE, that default cannot resolve the outer IP header’s destination because the specific prefix exists only inside the VRF. This forces the PE to drop the traffic, as it cannot find a valid next hop in the global table for recursive forwarding. On the Cisco SPCOR 350-501 exam, this scenario tests your understanding of MPLS L3VPN control versus data plane separation—a classic trap where engineers assume VPN routes automatically enable forwarding. Remember: VPN routes live in the VRF, but the global table must contain a route to the CE’s loopback or the PE-CE link subnet. Memory tip: “Global must host the ghost” — the global table must host the route for the next-hop IP that the VRF uses.
350-501 Networking Practice Question
This 350-501 practice question tests your understanding of networking. 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 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?
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 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.
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 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.
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 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.
Common exam traps
Common exam trap: answer the scenario, not the keyword
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.
Detailed technical explanation
How to think about this question
In MPLS L3VPN, the PE maintains separate routing tables: the global table for core/underlay routes and the VRF for customer VPN routes. When the CE sends traffic to a VPN prefix, the PE performs a recursive lookup: the VRF route points to a next-hop (often the remote PE), which must be reachable via the global routing table. If the global table lacks a route to that next-hop (e.g., the remote PE's loopback), the packet is dropped. This is commonly resolved by ensuring IGP (OSPF or IS-IS) or BGP advertises the PE loopbacks in the global table, or by using a default route in the global table that points to a valid next-hop.
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.
What to study next
Got this wrong? Here's your next step.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Networking — This question tests Networking — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: 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.
What should I do if I get this 350-501 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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