350-501 Networking Practice Question
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?
⚠ Common exam trap
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.
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
✓
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.
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.
- ✗
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.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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.