200-901 Cisco Platforms and Development Practice Question
This 200-901 practice question tests your understanding of cisco platforms and development. 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.
A DevOps team is implementing a CI/CD pipeline for network services using Cisco NSO (Network Services Orchestrator). They have a Python script that uses the NSO RESTCONF API to create a new VPN service instance. The script passes all integration tests in the staging environment, but when deployed to production, the 'POST' request to /api/run/vpn-service returns a 500 Internal Server Error. The team checks the NSO logs and finds the error 'Error: No such device: device-xr-1'. The production NSO instance manages multiple devices, and device-xr-1 is one of them. The team confirms device-xr-1 is reachable and managed by NSO in the production environment. What is the most likely cause of this error?
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
✓
The YANG service model expects a device name that does not match the device's name in NSO's device list.
The error 'No such device: device-xr-1' indicates that the YANG service model references a device name that does not match the actual device name configured in NSO's device list. Even though device-xr-1 is reachable and managed by NSO, the service model's 'device' leaf expects a specific name (e.g., 'xr-1' or 'device-xr-1-prod'), and the mismatch causes NSO to fail when trying to map the service to the device. This is a common issue when service YANG models are developed with hardcoded or environment-specific device names that differ between staging and production.
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 production NSO instance uses a different authentication method for device-xr-1.
Why it's wrong here
An authentication issue would produce a different error, such as 'Authentication failure'.
✗
The device configuration in NSO is out of sync; a sync-from is needed before service creation.
Why it's wrong here
Out-of-sync devices usually cause operational warnings, not a 'No such device' error.
✓
The YANG service model expects a device name that does not match the device's name in NSO's device list.
Why this is correct
The service template likely uses a device reference that conflicts with the actual device name.
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 production NSO instance has insufficient memory to handle the request.
Why it's wrong here
Memory issues would likely trigger other errors like 'Out of memory'.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the distinction between device reachability/authentication errors and device name mismatches in NSO service models, where candidates mistakenly assume a connectivity or sync issue when the actual problem is a YANG model reference mismatch.
Detailed technical explanation
How to think about this question
In NSO, service YANG models define 'leaf' nodes that reference devices via the 'tailf:ncs-device' extension, which enforces that the provided device name must match exactly one entry in NSO's device list. When a service is created via RESTCONF POST, NSO validates the device reference against its device tree; a mismatch triggers a 'No such device' error before any device communication occurs. This is a common pitfall when using environment-specific device names (e.g., 'device-xr-1-staging' vs 'device-xr-1-prod') or when the service model uses a different naming convention than the actual NSO device configuration.
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 practitioner preparing for the 200-901 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
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.
Cisco Platforms and Development — This question tests Cisco Platforms and Development — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The YANG service model expects a device name that does not match the device's name in NSO's device list. — The error 'No such device: device-xr-1' indicates that the YANG service model references a device name that does not match the actual device name configured in NSO's device list. Even though device-xr-1 is reachable and managed by NSO, the service model's 'device' leaf expects a specific name (e.g., 'xr-1' or 'device-xr-1-prod'), and the mismatch causes NSO to fail when trying to map the service to the device. This is a common issue when service YANG models are developed with hardcoded or environment-specific device names that differ between staging and production.
What should I do if I get this 200-901 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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