hardMultiple Choice
200-901 Practice Question: A DevOps team is implementing a CI/CD pipeline…
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?
⚠ Common exam trap
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.
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 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.
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
Authentication failure against device-xr-1 would surface as a connection or login error during sync or commit, not 'No such device'. It is tempting because staging and production commonly use different credentials, so authentication mismatches are a plausible explanation for environment-specific failures.
- ✗
The device configuration in NSO is out of sync; a sync-from is needed before service creation.
Why it's wrong here
An out-of-sync device still exists in NSO's device list, so service creation would not report 'No such device'; sync-from addresses configuration drift, not missing device references. It is tempting because sync issues are a frequent cause of service-creation failures in NSO deployments.
- ✓
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 500 error and 'No such device' message indicate the service instance's leaf references a device identifier absent from NSO's device list. A mismatch between the YANG model's expected device name and the actual name registered in NSO causes the mapping failure.
- ✗
The production NSO instance has insufficient memory to handle the request.
Why it's wrong here
Insufficient memory would produce resource-exhaustion or timeout errors, not the explicit 'No such device' message naming device-xr-1. It is tempting because production instances often run larger workloads than staging, so resource constraints are a common first suspicion when a request fails only after promotion.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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