350-501 MPLS and Segment Routing Practice Question
An engineer notices that MPLS VPN traffic is taking a suboptimal path because the MPLS label binding for the BGP next-hop loopback is missing. What is the most likely cause?
⚠ Common exam trap
Cisco often tests the distinction that LDP is required on core-facing interfaces for MPLS label distribution, not on PE-CE interfaces, and that the missing label binding is for the remote PE's loopback, not the local PE's route.
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.
MPLS VPN traffic relies on LDP to distribute labels for the BGP next-hop loopback addresses used in the VPNv4 route exchange. If LDP is not enabled on the core-facing interfaces, the local PE cannot obtain a label binding for the remote PE's loopback, causing the traffic to be forwarded via IP (without a label) or taking a suboptimal path. Enabling LDP on all core-facing interfaces resolves the missing label binding and restores optimal label-switched 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 remote PE is configured with a different VPN ID.
Why it's wrong here
VPN ID mismatch would affect route import/export, not label binding for the next-hop.
- ✗
The local PE does not have a route to its own loopback.
Why it's wrong here
A PE always has a route to its own loopback.
- ✓
LDP is not enabled on the core-facing interfaces.
Why this is correct
LDP on core interfaces is required to propagate labels for the loopback.
- ✗
LDP is not enabled on the PE-CE interface.
Why it's wrong here
PE-CE interfaces do not require LDP; labels for VPN routes are assigned via BGP.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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