The answer is a non-operational LSP between the PEs. When a CE can receive routes from its local PE but cannot ping the remote CE, the control plane is working—routes are being exchanged and installed in the VRF—but the data plane is failing. In an MPLS L3VPN, even with the correct route in the VRF, the packet must be label-switched across the provider core; if the label-switched path (LSP) is down, the PE cannot forward the MPLS-encapsulated packet to the remote PE, causing the ping to fail. This scenario directly tests your understanding of the separation between control plane (route exchange) and data plane (label forwarding) on the Cisco SPCOR 350-501 exam, a common trap where engineers focus on BGP or VRF misconfigurations instead. Remember the key distinction: if the route is present but ping fails, suspect the LSP, not the routing protocol. Memory tip: “Route in the VRF, but ping goes deaf—check the LSP, it’s the forwarding chief.”
350-501 MPLS and Segment Routing Practice Question
This 350-501 practice question tests your understanding of mpls and segment routing. 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 MPLS L3VPN issues on a PE router. The CE router is receiving routes from the PE, but the CE cannot ping the remote CE. The PE shows the remote prefix in its routing table and in the VRF. 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 LSP between the PEs is not operational
If the PE has the route in the VRF but the label-switched path (LSP) to the remote PE is down or nonexistent, the packet cannot be forwarded. The most common issue is that the LSP (e.g., via LDP or Segment Routing) is broken. Other options like wrong RT/RD or MTU would cause different symptoms.
Key principle: NAT direction and interface roles matter as much as the IP address mapping. Inside/outside designation controls which traffic is translated.
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/export is misconfigured
Why it's wrong here
If RT were wrong, the route would not appear in the VRF.
✗
The MTU on the core links is too small
Why it's wrong here
MTU issues typically cause fragmentation problems, not complete unreachability.
✗
The RD is different on the two PEs
Why it's wrong here
RD only needs to be unique per VPN per PE; different RDs are normal.
✓
The LSP between the PEs is not operational
Why this is correct
The LSP must be up to forward MPLS-encapsulated packets between PEs.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Static NAT maps one inside address to one outside address.
Common exam traps
Common exam trap: NAT rules depend on direction and matching traffic
NAT is not only about the public address. The inside/outside interface roles and the ACL or rule that matches traffic are just as important.
Detailed technical explanation
How to think about this question
NAT questions usually test address translation, overload/PAT behaviour, static mappings and whether the right traffic is being translated. Read the interface direction and address terms carefully.
KKey Concepts to Remember
Static NAT maps one inside address to one outside address.
PAT allows many inside hosts to share one public address using ports.
Inside local and inside global describe the private and translated addresses.
NAT ACLs identify traffic for translation, not always security filtering.
TExam Day Tips
→Identify inside and outside interfaces first.
→Check whether the scenario needs static NAT, dynamic NAT or PAT.
→Do not confuse NAT matching ACLs with normal packet-filtering intent.
Key takeaway
NAT direction and interface roles matter as much as the IP address mapping. Inside/outside designation controls which traffic is translated.
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.
Review the four NAT address types (inside local, inside global, outside local, outside global), PAT port overload, and static vs dynamic NAT use cases. Then practise related 350-501 NAT questions on configuration and troubleshooting.
MPLS and Segment Routing — This question tests MPLS and Segment Routing — Static NAT maps one inside address to one outside address..
What is the correct answer to this question?
The correct answer is: The LSP between the PEs is not operational — If the PE has the route in the VRF but the label-switched path (LSP) to the remote PE is down or nonexistent, the packet cannot be forwarded. The most common issue is that the LSP (e.g., via LDP or Segment Routing) is broken. Other options like wrong RT/RD or MTU would cause different symptoms.
What should I do if I get this 350-501 question wrong?
Review the four NAT address types (inside local, inside global, outside local, outside global), PAT port overload, and static vs dynamic NAT use cases. Then practise related 350-501 NAT questions on configuration and troubleshooting.
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?
Static NAT maps one inside address to one outside address.
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