The correct choice is to modify the subscription to use ON_CHANGE mode for the BGP neighbor paths. This is because gNMI ON_CHANGE subscriptions stream telemetry data only when a monitored value actually changes, ensuring every BGP route flap—a state transition from up to down or down to up—is captured in real time. In contrast, SAMPLE subscriptions poll the path at fixed intervals, and if a flap occurs and resolves between those intervals, the transient event is missed entirely, which explains why the NMS saw no update despite syslogs confirming the flaps. On the Cisco SPCOR 350-501 exam, this scenario tests your understanding of gNMI subscription modes and their impact on event-driven telemetry for service provider networks; a common trap is assuming SAMPLE is sufficient for stateful events like BGP flaps. Remember the memory tip: “ON_CHANGE for events, SAMPLE for trends”—if you need every state transition, always choose ON_CHANGE.
350-501 Automation and Assurance Practice Question
This 350-501 practice question tests your understanding of automation and assurance. 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.
You are responsible for network assurance for a Tier-1 ISP that has deployed model-driven telemetry using gNMI with ON_CHANGE subscriptions on all core routers. Recently, the NMS team reported that some BGP route flaps are not being captured in the telemetry data, even though the routers' syslogs show the flaps occurred. The telemetry subscription is for the path '/bgp/neighbors/neighbor/state/messages/received'. The NMS is using a gNMI collector that supports both ON_CHANGE and SAMPLE subscriptions. You suspect the issue is with the subscription configuration. Upon reviewing the router configuration, you see that the telemetry subscription uses the SAMPLE mode instead of ON_CHANGE. What is the most appropriate action to ensure all BGP route flap events are captured?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Modify the subscription to use ON_CHANGE mode for the BGP neighbor paths.
Option D is correct because the gNMI ON_CHANGE subscription mode is designed to stream telemetry updates only when the value of a subscribed path changes. Since the NMS is missing BGP flap events, the subscription must be using SAMPLE mode, which periodically polls the state and can miss transient events between sampling intervals. Changing the subscription to ON_CHANGE ensures that every state transition (e.g., BGP session up/down) is immediately pushed to the collector, capturing all flaps.
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.
✗
Configure the router to send syslogs to the NMS and parse them for BGP flaps.
Why it's wrong here
Syslogs are not structured telemetry and are not real-time.
✗
Add a second subscription with ON_CHANGE for the same paths to ensure redundancy.
Why it's wrong here
Redundant subscriptions are unnecessary and increase load.
✗
Change the subscription to SAMPLE with a 1-second interval to capture flaps more frequently.
Why it's wrong here
Even 1-second sampling may miss brief flaps.
✓
Modify the subscription to use ON_CHANGE mode for the BGP neighbor paths.
Why this is correct
ON_CHANGE ensures every state change is reported.
Related concept
Read the scenario before looking for a memorised answer.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the misconception that increasing SAMPLE frequency (e.g., 1-second interval) is sufficient to capture all events, when in fact only ON_CHANGE guarantees event-driven capture for state transitions.
Detailed technical explanation
How to think about this question
gNMI ON_CHANGE subscriptions rely on the device's internal event-driven notification mechanism (e.g., YANG data store change callbacks) to push updates only when a leaf value changes, ensuring zero data loss for state transitions. In contrast, SAMPLE subscriptions use periodic polling, which can miss short-lived events like BGP route flaps that last milliseconds. Real-world deployments often combine ON_CHANGE for critical state paths (e.g., BGP neighbor state) with SAMPLE for counters to balance accuracy and resource usage.
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.
Automation and Assurance — This question tests Automation and Assurance — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Modify the subscription to use ON_CHANGE mode for the BGP neighbor paths. — Option D is correct because the gNMI ON_CHANGE subscription mode is designed to stream telemetry updates only when the value of a subscribed path changes. Since the NMS is missing BGP flap events, the subscription must be using SAMPLE mode, which periodically polls the state and can miss transient events between sampling intervals. Changing the subscription to ON_CHANGE ensures that every state transition (e.g., BGP session up/down) is immediately pushed to the collector, capturing all flaps.
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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