350-501 Automation and Assurance Practice Question
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?
⚠ Common exam trap
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.
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
✓
Modify the subscription to use ON_CHANGE mode for the BGP neighbor paths.
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.
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.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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