- A
The subscription is configured with both on-change and periodic updates
Why wrong: On-change subscriptions do not support periodic updates; if both are configured, it would be an error.
- B
The YANG path includes multiple leafs that change together, each triggering a separate update
On-change subscriptions send an update for each leaf that changes, so multiple leaf changes from a single event cause duplicate updates.
- C
The collector is sending acknowledgments that cause the router to resend data
Why wrong: gRPC does not use acknowledgments for telemetry data; it is a one-way stream.
- D
The engineer used JSON encoding which causes larger payloads and fragmentation
Why wrong: Encoding does not cause duplicate updates; fragmentation would not create duplicates.
Quick Answer
The answer is that the YANG path includes multiple leafs that change together, each triggering a separate update. When an OSPF neighbor transitions from FULL to DOWN, the on-change subscription monitors every leaf under the specified YANG model path, such as state, last-up-time, and neighbor-address. Because these leafs change simultaneously, each one independently fires an on-change telemetry update, causing the collector to receive duplicate updates for the same event. On the ENCOR 350-401 exam, this question tests your understanding of how on-change subscriptions differ from periodic subscriptions—specifically, that they react to individual leaf changes rather than a single event. A common trap is assuming the sample-interval applies, but on-change subscriptions ignore it entirely. Remember the memory tip: “One event, many leafs—each leaf sends its own brief.”
CCNP Model-Driven Telemetry Practice Question
This 350-401 practice question tests your understanding of model-driven telemetry. 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 engineer is configuring model-driven telemetry on a Cisco IOS-XE router to stream OSPF neighbor state changes. The engineer uses the YANG model 'Cisco-IOS-XE-ospf-oper' and creates an on-change subscription. After testing, the engineer notices that the telemetry data is being sent, but the collector is receiving duplicate updates for the same OSPF neighbor state change. What is the most likely cause of these duplicate updates?
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 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
The YANG path includes multiple leafs that change together, each triggering a separate update
The correct answer is that the YANG path includes multiple leafs that change simultaneously, causing multiple updates for the same event. For example, when an OSPF neighbor goes from FULL to DOWN, multiple leafs (state, last-up-time, etc.) change, and each change triggers an on-change update. The other options are incorrect because the sample-interval is not used in on-change subscriptions; the collector is not causing duplicates; and the encoding does not cause duplicates.
Key principle: OSPF neighbour adjacency depends on matching area, hello/dead timers, network type, and authentication — IP reachability alone is not enough.
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 subscription is configured with both on-change and periodic updates
Why it's wrong here
On-change subscriptions do not support periodic updates; if both are configured, it would be an error.
- ✓
The YANG path includes multiple leafs that change together, each triggering a separate update
Why this is correct
On-change subscriptions send an update for each leaf that changes, so multiple leaf changes from a single event cause duplicate updates.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
OSPF neighbours must agree on key parameters.
- ✗
The collector is sending acknowledgments that cause the router to resend data
Why it's wrong here
gRPC does not use acknowledgments for telemetry data; it is a one-way stream.
- ✗
The engineer used JSON encoding which causes larger payloads and fragmentation
Why it's wrong here
Encoding does not cause duplicate updates; fragmentation would not create duplicates.
Common exam traps
Common exam trap: OSPF can fail even when IP connectivity looks correct
OSPF neighbour formation depends on matching areas, timers, network type, authentication and passive-interface behaviour. Do not choose an answer only because the devices can ping.
Detailed technical explanation
How to think about this question
OSPF questions usually test the details that control adjacency and route selection. Read the neighbour state, area, router ID and interface configuration before deciding what is wrong.
KKey Concepts to Remember
- OSPF neighbours must agree on key parameters.
- Router ID selection can affect neighbour relationships and LSDB output.
- OSPF cost influences the preferred path.
- A route can appear in OSPF information but not become the installed route.
TExam Day Tips
- Check area mismatch first when OSPF adjacency fails.
- Review passive interfaces when a network is advertised but no neighbour forms.
- Use show ip ospf neighbor and show ip route clues carefully.
Key takeaway
OSPF neighbour adjacency depends on matching area, hello/dead timers, network type, and authentication — IP reachability alone is not enough.
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.
Review OSPF neighbour requirements — matching area type, hello and dead timers, network type, stub flags, and authentication. Study show ip ospf neighbor states (INIT, 2-WAY, FULL). Then practise related 350-401 OSPF questions on adjacency and route selection.
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Model-Driven Telemetry — study guide chapter
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Model-Driven Telemetry practice questions
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FAQ
Questions learners often ask
What does this 350-401 question test?
Model-Driven Telemetry — This question tests Model-Driven Telemetry — OSPF neighbours must agree on key parameters..
What is the correct answer to this question?
The correct answer is: The YANG path includes multiple leafs that change together, each triggering a separate update — The correct answer is that the YANG path includes multiple leafs that change simultaneously, causing multiple updates for the same event. For example, when an OSPF neighbor goes from FULL to DOWN, multiple leafs (state, last-up-time, etc.) change, and each change triggers an on-change update. The other options are incorrect because the sample-interval is not used in on-change subscriptions; the collector is not causing duplicates; and the encoding does not cause duplicates.
What should I do if I get this 350-401 question wrong?
Review OSPF neighbour requirements — matching area type, hello and dead timers, network type, stub flags, and authentication. Study show ip ospf neighbor states (INIT, 2-WAY, FULL). Then practise related 350-401 OSPF questions on adjacency and route selection.
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?
OSPF neighbours must agree on key parameters.
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Last reviewed: Jun 18, 2026
This 350-401 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 350-401 exam.
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