350-501 Networking Practice Question
A service provider has recently deployed MPLS L3VPN to provide IP connectivity to multiple enterprise customers. One customer reports that they cannot reach a remote site that is connected to a different PE router. The engineer checks the BGP VPNv4 table on the customer's PE and sees the route for the remote site, but the next-hop is unreachable. The interface between the PE and P routers is up/up, and IGP reachability to the PE's loopback is fine. What is the most likely cause? Consider that the network uses LDP for label distribution and OSPF as the IGP.
⚠ Common exam trap
The trap here is that candidates often focus on BGP configuration (like next-hop-self) or VRF import/export when the route is present but unreachable, missing the fundamental MPLS label distribution issue that LDP must be enabled on all transit interfaces for end-to-end LSPs.
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
✓
MPLS LDP is not enabled on the interface connecting to the P router
Since the interface between the PE and P routers is up/up and IGP reachability to the PE's loopback is fine, the issue is that MPLS LDP is not enabled on that interface. Without LDP, the PE cannot advertise a label for its loopback to the P router, so the P router cannot push the correct label for packets destined to the remote PE. This makes the BGP VPNv4 next-hop unreachable even though the route itself is present in the table.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
MPLS LDP is not enabled on the interface connecting to the P router
Why this is correct
LDP must be enabled on the interface to exchange labels for the loopback route.
- ✗
BGP next-hop-self is not configured on the PE
Why it's wrong here
Next-hop-self changes the next-hop in BGP updates, but the next-hop is already the remote PE.
- ✗
The VRF route-target import/export is misconfigured
Why it's wrong here
Misconfigured RT would prevent the route from being in the VRF.
- ✗
The PE router does not have an LSP to the remote PE
Why it's wrong here
LSP existence is a consequence of LDP; this answer is less specific.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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