350-501 Architecture Practice Question
You are a network engineer at a service provider that offers L3VPN services. A customer complains that their VPN traffic is intermittently dropping. The network uses OSPF as the IGP, LDP for label distribution, and MP-BGP for VPNv4 route exchange. The PE routers are connected to two core routers (P1 and P2) in a redundant design. Upon investigation, you find that the BGP next-hop for some VPN prefixes is set to the loopback interface of the remote PE, but the route to that loopback is sometimes unreachable due to a flapping link between P1 and P2. The IGP converges, but the BGP prefixes take longer to recover. What is the most likely cause of the intermittent drops?
⚠ Common exam trap
Cisco often tests the misconception that BGP automatically reacts to IGP changes instantly, when in fact without NHT, BGP relies on periodic scanning, causing a delay that leads to traffic drops during transient IGP failures.
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
✓
BGP next-hop tracking is not enabled, causing BGP to rely on IGP convergence only
The intermittent drops occur because BGP next-hop tracking (NHT) is not enabled. Without NHT, BGP relies solely on IGP convergence to detect reachability changes for the BGP next-hop (the remote PE loopback). When the link between P1 and P2 flaps, OSPF converges quickly, but BGP does not immediately re-evaluate its next-hop reachability; it waits for the next BGP scan interval (default 60 seconds) or until a BGP update is received. This delay causes the VPN prefixes to remain installed with an unreachable next-hop, leading to traffic drops until BGP eventually withdraws or re-advertises the routes.
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 OSPF network type is incorrectly set to point-to-point
Why it's wrong here
OSPF network type does not affect BGP next-hop tracking.
- ✗
The BGP timers are set too low, causing premature route withdrawal
Why it's wrong here
Low timers would cause faster convergence, not slower.
- ✓
BGP next-hop tracking is not enabled, causing BGP to rely on IGP convergence only
Why this is correct
NHT allows BGP to react quickly to IGP next-hop changes.
- ✗
The LDP session between the PEs is down
Why it's wrong here
LDP is used for label distribution, not for BGP next-hop resolution.
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JA
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.