350-501 MPLS and Segment Routing Practice Question
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?
⚠ Common exam trap
Cisco often tests the distinction between control plane and data plane failures; the trap here is that candidates see routes in the VRF and assume the data path is working, but they overlook the requirement for a functional LSP to forward MPLS-encapsulated traffic between PEs.
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 LSP between the PEs is not operational
The CE can receive routes from the PE (indicating that the VRF and route-target import/export are functioning correctly), but the CE cannot ping the remote CE. This points to a failure in the data plane between the PEs. If the LSP (Label Switched Path) between the PEs is not operational, MPLS labels cannot be swapped or forwarded correctly, causing packets to be dropped even though the control plane (routing table and VRF) shows the remote 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 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.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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