Quick Answer
The answer is ON_CHANGE. This gNMI subscription mode ensures that all state changes are captured because the target device pushes a telemetry update immediately upon any state transition, guaranteeing that no transient events are missed between collection intervals. In contrast, SAMPLE mode sends periodic snapshots at a fixed rate, which can overlook rapid or short-lived changes that occur between samples. On the Cisco SPCOR / CCNP Service Provider Core 350-501 exam, this concept tests your understanding of telemetry reliability and the trade-offs between completeness and bandwidth; a common trap is assuming SAMPLE mode with a high frequency is sufficient, but only ON_CHANGE provides deterministic capture of every state change. Remember the memory tip: “ON_CHANGE = ON the moment of CHANGE,” ensuring you never miss a beat in your telemetry stream.
This 350-501 practice question tests your understanding of automation and assurance. Read the scenario carefully and evaluate each option against the stated constraints before committing to an answer. 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.
A network operator uses gRPC Network Management Interface (gNMI) to collect telemetry data from routers. They notice that some updates are missing. Which gNMI mode should be used to ensure that all state changes are captured?
ON_CHANGE
Sends updates only when a value changes, capturing all changes.
TARGET_DEFINED
Why wrong: Not a standard gNMI mode.
POLL
Why wrong: Polling is not a gNMI subscription mode.
SAMPLE
Why wrong: Periodic sampling may miss changes between intervals.
Answer choices
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
ON_CHANGE
ON_CHANGE mode in gNMI ensures that the target device sends a telemetry update immediately whenever a state change occurs, guaranteeing that no updates are missed. This is in contrast to SAMPLE mode, which only sends periodic snapshots and can miss transient changes between intervals. Therefore, to capture all state changes, ON_CHANGE is the correct subscription mode.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
For each option: why learners choose it and why it is or isn't the right answer here.
ON_CHANGE
Why this is correct
Sends updates only when a value changes, capturing all changes.
Related concept
Read the scenario before looking for a memorised answer.
TARGET_DEFINED
Why it's wrong here
Not a standard gNMI mode.
POLL
Why it's wrong here
Polling is not a gNMI subscription mode.
SAMPLE
Why it's wrong here
Periodic sampling may miss changes between intervals.
Common exam traps
Cisco often tests the misconception that SAMPLE mode with a very short interval is sufficient to capture all changes, but the trap is that SAMPLE can still miss state changes that occur and revert between samples, whereas ON_CHANGE guarantees delivery of every transition.
Detailed technical explanation
gNMI uses a subscription model defined in the gNMI specification (RFC-like via OpenConfig), where ON_CHANGE subscriptions rely on the target device's internal event-driven mechanism to push updates only when a configured path's value changes, often using YANG-modeled data. This mode is critical for capturing fast transient states (e.g., interface flaps or BGP route changes) that could be missed by SAMPLE intervals, even with sub-second sampling. In real-world deployments, ON_CHANGE reduces bandwidth and processing overhead compared to high-frequency SAMPLE, but requires the target to support event-driven telemetry, which is common in modern IOS XR and NX-OS devices.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
A practitioner preparing for the 350-501 exam encounters this exact type of scenario on the job. The correct answer here is not the most general option — it is the best answer for the specific constraint described. Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option. Real exam questions reward reading the full scenario before eliminating options, because the constraint defines which answer fits.
What to study next
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Automation and Assurance — study guide chapter
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FAQ
Automation and Assurance — This question tests Automation and Assurance — Read the scenario before looking for a memorised answer..
The correct answer is: ON_CHANGE — ON_CHANGE mode in gNMI ensures that the target device sends a telemetry update immediately whenever a state change occurs, guaranteeing that no updates are missed. This is in contrast to SAMPLE mode, which only sends periodic snapshots and can miss transient changes between intervals. Therefore, to capture all state changes, ON_CHANGE is the correct subscription mode.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Read the scenario before looking for a memorised answer.
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Last reviewed: Jun 11, 2026
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.
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