The answer is a missing or mismatched SSH host key in the controller’s known_hosts file. This is the most likely cause because NETCONF over SSH (RFC 6242) relies on the SSH transport layer to authenticate the remote device; during the SSH handshake, the controller compares the router’s host key against its known_hosts file, and if the key is absent or incorrect, the session fails before any NETCONF capabilities can be exchanged. On the Cisco SPCOR 350-501 exam, this scenario tests your understanding that NETCONF is not a standalone protocol but rides on SSH, so connectivity (ping) and service configuration (port 830) are necessary but insufficient—the host key trust is the gatekeeper. A common trap is assuming the issue is a firewall or ACL, but the key mismatch is the subtle culprit. Memory tip: think “SSH first, NETCONF second”—if the handshake fails, the session never starts.
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 is troubleshooting a NETCONF session that fails to establish between a controller and a router. The router supports NETCONF over SSH on port 830. The controller can reach the router but the session fails. What is the most likely cause?
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 SSH host key of the router is not in the controller's known_hosts file
The most likely cause is that the SSH host key of the router is not in the controller's known_hosts file. NETCONF over SSH (RFC 6242) requires SSH transport, and the controller must authenticate the router's SSH host key during session establishment. If the host key is missing or mismatched, the SSH handshake fails, preventing the NETCONF session from starting, even though the router is reachable and NETCONF is enabled.
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 router's NETCONF capability is disabled
Why it's wrong here
If disabled, the router wouldn't listen on port 830.
✓
The SSH host key of the router is not in the controller's known_hosts file
Why this is correct
SSH host key verification failure can cause session failure.
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 controller is using the wrong port (e.g., 22)
Why it's wrong here
The engineer knows port 830 is correct.
✗
The router does not support YANG models
Why it's wrong here
NETCONF sessions can establish even without YANG models, but models are needed for data access.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the distinction between transport-layer failures (SSH host key) and application-layer failures (NETCONF capability or YANG support), leading candidates to incorrectly choose options related to NETCONF configuration rather than SSH authentication.
Detailed technical explanation
How to think about this question
NETCONF over SSH uses the SSH transport layer as defined in RFC 6242, where the client (controller) must verify the server's (router's) host key against its known_hosts file. If the host key is not present or has changed, SSH will abort the connection with a host key verification failure, which is a common issue in automated environments where controllers are not pre-configured with device SSH keys. In real-world scenarios, this is often resolved by manually accepting the host key or using SSH key distribution mechanisms.
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 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
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 SSH host key of the router is not in the controller's known_hosts file — The most likely cause is that the SSH host key of the router is not in the controller's known_hosts file. NETCONF over SSH (RFC 6242) requires SSH transport, and the controller must authenticate the router's SSH host key during session establishment. If the host key is missing or mismatched, the SSH handshake fails, preventing the NETCONF session from starting, even though the router is reachable and NETCONF is enabled.
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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