The answer is that the devices are not in sync with NSO, which is the most likely cause of the deployment failure. When NSO attempts to deploy a service, it performs a pre-deployment synchronization check against its configuration database (CDB); if a device’s running configuration differs from what NSO expects—meaning it is out of sync—NSO will refuse to push the new configuration and report a “failed to reach devices” error, even though SSH and NETCONF connectivity are fine. This safety mechanism prevents configuration conflicts and overwriting unmanaged changes, a key concept tested on the Cisco SPCOR / CCNP Service Provider Core 350-501 exam. A common trap is assuming the error indicates a network reachability problem, but the real issue is configuration drift. Remember the memory tip: “Sync before you deploy, or NSO will say nay.”
350-501 Automation and Assurance Practice Question
This 350-501 practice question tests your understanding of automation and assurance. 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 network engineer at a service provider is using Cisco NSO to automate the provisioning of VLANs on thousands of access devices. The engineer creates a service using a custom YANG model and deploys it to a set of devices. However, the deployment fails with a 'failed to reach devices' error for some devices, while others succeed. The engineer checks device connectivity and confirms all devices are reachable via SSH and NETCONF. The engineer also verifies that the NSO device list is accurate and includes all target devices. What is the most likely cause of the failure?
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.
The service model uses an unsupported feature on those devices.
Why wrong: An unsupported feature would result in a validation error, not a connectivity failure.
B
The devices are not in sync with NSO.
Out-of-sync devices prevent NSO from deploying services on them, and the error may manifest as 'failed to reach' because NSO cannot reconcile the configuration.
C
The devices have insufficient memory to accept the configuration.
Why wrong: Insufficient memory would cause a different error during configuration push, not a 'failed to reach' error.
D
The NSO package is not loaded on those devices.
Why wrong: NSO packages are loaded on the NSO server, not on the devices; this would not affect reachability.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The devices are not in sync with NSO.
When NSO deploys a service, it first checks whether the target devices are in sync with the NSO CDB (configuration database). If a device is out of sync (e.g., its running configuration differs from what NSO expects), NSO will refuse to push the new service configuration and will report a 'failed to reach devices' error, even though the device is reachable via SSH/NETCONF. This is a safety mechanism to prevent configuration conflicts or overwriting unmanaged changes.
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 service model uses an unsupported feature on those devices.
Why it's wrong here
An unsupported feature would result in a validation error, not a connectivity failure.
✓
The devices are not in sync with NSO.
Why this is correct
Out-of-sync devices prevent NSO from deploying services on them, and the error may manifest as 'failed to reach' because NSO cannot reconcile the configuration.
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 devices have insufficient memory to accept the configuration.
Why it's wrong here
Insufficient memory would cause a different error during configuration push, not a 'failed to reach' error.
✗
The NSO package is not loaded on those devices.
Why it's wrong here
NSO packages are loaded on the NSO server, not on the devices; this would not affect reachability.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the misconception that 'failed to reach devices' always indicates a network connectivity problem, when in fact it can be caused by NSO's synchronization check failing on a reachable device.
Detailed technical explanation
How to think about this question
NSO uses a 'sync-from' operation to reconcile the device's actual configuration with its CDB. If a device is out of sync, NSO's deployment engine will abort the service push to avoid overwriting unmanaged changes. The 'failed to reach devices' error is a generic message that NSO uses when it cannot proceed with the deployment due to synchronization issues, not necessarily a network connectivity failure. Engineers can check device sync status with the 'devices device <name> check-sync' command.
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 help-desk technician troubleshoots why a newly connected PC cannot reach shared printers on the same floor. The cable is good, the switch port is active, but the PC is in VLAN 20 and the printers are in VLAN 10. The uplink trunk only allows VLAN 10. A trunk being up does not mean every VLAN crosses it.
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.
Automation and Assurance — This question tests Automation and Assurance — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The devices are not in sync with NSO. — When NSO deploys a service, it first checks whether the target devices are in sync with the NSO CDB (configuration database). If a device is out of sync (e.g., its running configuration differs from what NSO expects), NSO will refuse to push the new service configuration and will report a 'failed to reach devices' error, even though the device is reachable via SSH/NETCONF. This is a safety mechanism to prevent configuration conflicts or overwriting unmanaged changes.
What should I do if I get this 350-501 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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