The answer is that BGP next-hop tracking (NHT) is not enabled, which causes the intermittent VPN drops. Without NHT, BGP relies solely on IGP convergence to detect changes in reachability for the BGP next-hop—in this case, the remote PE’s loopback. When the link between P1 and P2 flaps, OSPF converges quickly, but BGP does not immediately re-evaluate next-hop reachability; it waits for the default 60-second BGP scan interval, leaving VPN prefixes installed with an unreachable next-hop and dropping traffic until the next scan or a BGP update occurs. On the Cisco SPCOR 350-501 exam, this scenario tests your understanding of how BGP interacts with the IGP in an MPLS L3VPN environment—a common trap is assuming IGP convergence alone fixes BGP reachability. Remember the memory tip: “NHT makes BGP jump, not wait for the scan.”
350-501 Architecture Practice Question
This 350-501 practice question tests your understanding of architecture. Match the stated requirement to the specific cloud service, access model, or configuration option — many options are valid in isolation but not for this scenario. 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.
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?
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
✓
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.
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 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.
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 LDP session between the PEs is down
Why it's wrong here
LDP is used for label distribution, not for BGP next-hop resolution.
Common exam traps
Common exam trap: answer the scenario, not the keyword
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.
Detailed technical explanation
How to think about this question
BGP next-hop tracking (NHT) is a feature that allows BGP to subscribe to routing information base (RIB) notifications for changes to the next-hop address. When enabled, BGP immediately reacts to IGP reachability changes for the next-hop, triggering a scan and route re-evaluation without waiting for the default 60-second BGP scan interval. In service provider networks with flapping core links, NHT is critical to minimize traffic blackholing; it can be verified with 'show bgp nexthop' and configured globally with 'bgp nexthop trigger enable'.
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: 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.
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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