The answer is to clear LDP neighbor sessions on P2 to re-initiate label exchange. This is correct because the show mpls forwarding-table output on P2 reveals a valid label for the VPN prefix but a null outgoing interface, which indicates a corrupted or incomplete LDP label mapping for the BGP next-hop—likely caused by the new link between P2 and P3 failing to trigger proper label advertisement. In the context of the Cisco SPCOR 350-501 exam, this scenario tests your ability to differentiate between a BGP routing issue and an MPLS LDP label missing blackhole; the trap is that engineers often chase OSPF or BGP problems when the real culprit is a stale LDP binding. Remember that a label with a null outgoing interface is LDP’s way of saying “I have the label, but I don’t know where to send it”—clearing the LDP session forces a fresh label exchange without disrupting the IGP or BGP. Memory tip: “Null out means LDP doubt—clear the session, sort it out.”
350-501 Networking Practice Question
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 operates a large MPLS VPN network using OSPF as the IGP and LDP for label distribution. The PE routers (PE1, PE2, PE3) are connected to a core of P routers. Recently, a new link was added between P2 and P3. After the link came up, the engineering team noticed that several VPN routes that were previously reachable via PE2 are now being blackholed when traffic is sent from PE1 to those prefixes. The teams verify that the VPNv4 routes are present in the BGP table on PE1 with valid next-hops, but traffic fails. The traceroute from PE1 to the CE behind PE2 stops at P2. The show mpls forwarding-table on P2 shows the correct label for the VPN prefix, but the outgoing interface is null. Which action should the engineer take to resolve the issue without causing additional disruption?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Clear LDP neighbor sessions on P2 to re-initiate label exchange.
The issue is that P2 has a label for the VPN prefix but a null outgoing interface, indicating an LDP label mapping problem. Clearing LDP neighbor sessions on P2 forces re-establishment of LDP sessions and re-exchange of label bindings, which should resolve the missing or incorrect label mapping for the BGP next-hop without disrupting other services.
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.
✓
Clear LDP neighbor sessions on P2 to re-initiate label exchange.
Why this is correct
Correct. This forces LDP to re-establish and exchange labels, likely resolving the missing label.
Related concept
Read the scenario before looking for a memorised answer.
✗
Add a static route on P2 for the BGP next-hop pointing to Null0.
Why it's wrong here
Incorrect. This would drop traffic intentionally.
✗
Shut down the new link between P2 and P3.
Why it's wrong here
Incorrect. This would remove the new link but does not fix the underlying issue.
✗
Clear BGP sessions on PE1 to force re-advertisement of VPN routes.
Why it's wrong here
Incorrect. BGP is working; the issue is LDP label distribution.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the misconception that clearing BGP sessions (Option D) fixes MPLS forwarding issues, but the real problem is at the LDP label distribution layer, not the BGP VPN route advertisement.
Detailed technical explanation
How to think about this question
In MPLS LDP environments, when a new link is added, LDP must exchange label bindings for all FECs reachable via that link. If LDP fails to update the label information base (LIB) correctly, the forwarding table may show a label but with a null outgoing interface (e.g., 'Pop tag' or 'Untagged'), causing traffic to be dropped. This can occur if LDP sessions are not fully established or if label withdrawal/advertisement is delayed, a scenario often seen when OSPF converges faster than LDP.
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: Clear LDP neighbor sessions on P2 to re-initiate label exchange. — The issue is that P2 has a label for the VPN prefix but a null outgoing interface, indicating an LDP label mapping problem. Clearing LDP neighbor sessions on P2 forces re-establishment of LDP sessions and re-exchange of label bindings, which should resolve the missing or incorrect label mapping for the BGP next-hop without disrupting other services.
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.
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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