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 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.
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
✓
MPLS LDP is not enabled on the interface connecting to the P router
The correct answer is A. 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.
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.
✓
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.
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.
✗
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.
Common exam traps
Common exam trap: answer the scenario, not the keyword
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.
Detailed technical explanation
How to think about this question
In an MPLS L3VPN network, LDP is used to distribute labels for IGP routes, including the loopback addresses of PEs. When LDP is not enabled on a link, the P router cannot learn the label binding for the remote PE's loopback, so it cannot perform label switching for VPN traffic. This is often verified with the 'show mpls ldp neighbor' and 'show mpls forwarding-table' commands; a missing LDP session on the interface is a common misconfiguration after a new deployment.
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 network engineer at a university connects two campus buildings via a fibre link. Both routers run OSPF, but no adjacency forms — even though both routers can ping each other. The engineer finds one router is in area 0 and the other in area 1. OSPF adjacency requires matching area numbers, hello/dead timers, and network type. IP reachability alone is not enough.
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: MPLS LDP is not enabled on the interface connecting to the P router — The correct answer is A. 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.
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.
About these practice questions
Courseiva creates original exam-style practice questions with explanations and wrong-answer analysis. It does not publish real exam questions, exam dumps, or protected exam content. Learn why practice questions differ from exam dumps →
Share a tip, memory trick, or ask about the reasoning behind this question. Do not post real exam questions, leaked content, braindumps, or copyrighted exam material. Comments are moderated and may be removed without notice.
This 350-501 practice question is part of Courseiva's free Cisco certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the 350-501 exam.
Question Discussion
Share a tip, memory trick, or ask about the reasoning behind this question. Do not post real exam questions, leaked content, braindumps, or copyrighted exam material. Comments are moderated and may be removed without notice.
Sign in to join the discussion.