The answer is that LDP is not enabled on the core interfaces between PE1 and PE2. This is correct because MPLS L3VPN relies on LDP to distribute labels for the transport LSP that carries traffic across the service provider core; without LDP sessions between the PEs, the MPLS forwarding path is broken, so even if BGP VPNv4 routes are exchanged, the CE routers cannot ping each other since no label-switched path exists for the data plane. On the Cisco SPCOR 350-501 exam, this scenario tests your understanding of the separation between the control plane (BGP VPNv4) and the data plane (LDP label distribution), and a common trap is assuming that BGP alone is sufficient for end-to-end connectivity. Remember the memory tip: “BGP builds the map, LDP builds the road”—without LDP, the MPLS highway between PEs is missing, and CE traffic cannot travel.
350-501 Architecture Practice Question
This 350-501 practice question tests your understanding of architecture. 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 customer reports that CE routers attached to PE1 and PE2 in the same VRF cannot ping each other. Based on the exhibit, 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 interfaces between PE1 and PE2
B is correct because LDP must be enabled on the core interfaces between PE1 and PE2 to establish LDP sessions, which are required to exchange MPLS labels for the transport LSP. Without LDP, the MPLS forwarding path between the PEs is broken, preventing CE-to-CE ping even if BGP VPNv4 routes are correctly advertised.
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.
✗
Missing address-family ipv4 for BGP neighbor
Why it's wrong here
The VPNv4 address-family is activated; the ipv4 unicast is not required for VPNv4.
✓
LDP is not enabled on the core interfaces between PE1 and PE2
Why this is correct
LDP is required to distribute labels for BGP next-hop reachability; without it, MPLS forwarding fails.
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.
✗
Mismatched route distinguisher (RD) values on PE1 and PE2
Why it's wrong here
Both have RD 65000:1, so they match.
✗
Route target (RT) import/export mismatch
Why it's wrong here
Both have same RT 65000:100 for export and import.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the distinction between control-plane issues (BGP VPNv4, RT, RD) and data-plane issues (LDP, MPLS forwarding), leading candidates to focus on route advertisement problems when the actual fault is at the MPLS transport layer.
Detailed technical explanation
How to think about this question
In MPLS L3VPN, LDP distributes labels for the IGP routes (e.g., loopback interfaces) that form the BGP next-hop reachability between PEs. If LDP is missing on core interfaces, the PEs cannot push the correct transport label for VPN packets, causing MPLS forwarding failure even though BGP VPNv4 routes are present. A common verification command is 'show mpls ldp neighbor' to confirm LDP sessions are up.
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.
Architecture — This question tests Architecture — 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 interfaces between PE1 and PE2 — B is correct because LDP must be enabled on the core interfaces between PE1 and PE2 to establish LDP sessions, which are required to exchange MPLS labels for the transport LSP. Without LDP, the MPLS forwarding path between the PEs is broken, preventing CE-to-CE ping even if BGP VPNv4 routes are correctly advertised.
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.
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Question Discussion
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