The answer is that LDP is not enabled on the core-facing interfaces of PE1 or the P routers. This is the most likely cause because, in an MPLS L3VPN, the VPNv4 route for 10.2.2.0/24 is received via MP-BGP but cannot be installed into the VRF routing table until the recursive lookup on the BGP next-hop (192.168.1.2) succeeds. Without a label in the MPLS forwarding table for that next-hop, the route remains hidden, as seen when the show ip route vrf CUSTOMER command does not display the prefix. On the Cisco CCNP ENARSI 300-410 exam, this scenario tests your understanding of the MPLS control plane dependency between LDP and BGP—a common trap is to assume the issue is with MP-BGP peering or route-target mismatches, but the real culprit is the missing LDP label for the BGP next-hop. Remember the memory tip: "No label, no route—LDP must label the next-hop before BGP can install."
300-410 MPLS L3VPN Practice Question
This 300-410 practice question tests your understanding of mpls l3vpn. 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.
An engineer is troubleshooting an MPLS L3VPN where CE1 (10.1.1.0/24) cannot reach CE2 (10.2.2.0/24). The PE routers have MP-BGP peering and the VRF is configured with route-target import 100:100. On PE1, the show ip bgp vpnv4 vrf CUSTOMER command shows the route for 10.2.2.0/24 with a next-hop of 192.168.1.2 (the PE2 loopback), but the show ip route vrf CUSTOMER command does not have this route. The show mpls forwarding-table on PE1 does not show a label for 192.168.1.2. 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
✓
LDP is not enabled on the core-facing interfaces of PE1 or the P routers.
The VPNv4 route is received but not installed in the VRF routing table because the recursive lookup fails: the next-hop (PE2 loopback) is not reachable via LDP. Without an LDP label for the BGP next-hop, the route cannot be installed.
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.
✗
The VRF route-target import is missing on PE2.
Why it's wrong here
Incorrect because the route is received on PE1, so import is working.
✓
LDP is not enabled on the core-facing interfaces of PE1 or the P routers.
Why this is correct
Correct: Without LDP, there is no label for the BGP next-hop, preventing route installation.
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.
✗
The MP-BGP session is not using the loopback interface.
Why it's wrong here
Incorrect because the route shows a next-hop of the loopback, indicating the session is using it.
✗
The VRF on PE1 has the wrong route-target export.
Why it's wrong here
Incorrect because the issue is with route installation, not export.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Many certification questions include familiar terms but test a specific constraint. Read the exact wording before choosing an answer that is generally true but wrong for this case.
Trap categories for this question
Command / output trap
Incorrect because the route shows a next-hop of the loopback, indicating the session is using it.
Detailed technical explanation
How to think about this question
This question should be treated as a scenario, not a definition check. Identify the problem, the constraint and the best action. Then compare each option against those facts.
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.
Use explanations to understand the rule behind the answer.
TExam Day Tips
→Underline the problem statement mentally.
→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 practitioner preparing for the 300-410 exam encounters this exact type of scenario on the job. The correct answer here is not the most general option — it is the best answer for the specific constraint described. Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option. Real exam questions reward reading the full scenario before eliminating options, because the constraint defines which answer fits.
Related glossary terms
Concepts from this question explained
These glossary pages explain the core terms tested in this 300-410 question in full detail.
Identify which 300-410 exam domain this question belongs to, then review the specific concept being tested. Practise related questions in that domain and focus on understanding why each wrong answer is tempting — not just why the correct answer is right.
MPLS L3VPN — This question tests MPLS L3VPN — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: LDP is not enabled on the core-facing interfaces of PE1 or the P routers. — The VPNv4 route is received but not installed in the VRF routing table because the recursive lookup fails: the next-hop (PE2 loopback) is not reachable via LDP. Without an LDP label for the BGP next-hop, the route cannot be installed.
What should I do if I get this 300-410 question wrong?
Identify which 300-410 exam domain this question belongs to, then review the specific concept being tested. Practise related questions in that domain and focus on understanding why each wrong answer is tempting — not just why the correct answer is right.
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.
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