The answer is that a 400 Bad Request error from RESTCONF is most likely caused by a JSON payload containing incorrect data types or missing mandatory leafs. This occurs because RESTCONF validates the incoming YANG-modeled data against the device’s schema; if the JSON structure does not match the expected data types—such as sending a string where an integer is required—or omits a leaf defined as mandatory in the YANG module, the server rejects the request with a 400 status. On the Cisco SPCOR / CCNP Service Provider Core 350-501 exam, this question tests your understanding of RESTCONF error codes and their mapping to client-side mistakes, often as a distractor from authentication (401) or unsupported modules (404). A common trap is confusing a 400 with a 404 when the URI is wrong, but remember: 400 means “you sent bad data,” not “I can’t find that resource.” Memory tip: “400 = Four hundred = Faulty payload” — always double-check your JSON syntax and required fields first.
350-501 Automation and Assurance Practice Question
This 350-501 practice question tests your understanding of automation and assurance. This is a configuration task: choose the command set that satisfies every stated requirement. Small differences — like 'secret' vs 'password' or 'transport input ssh' vs 'all' — change whether the answer is correct. 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.
An engineer is using RESTCONF to configure an interface on a Cisco IOS-XE device. The request returns a 400 Bad Request error. 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 JSON payload contains incorrect data types or missing mandatory leafs
A 400 Bad Request typically indicates a client error, such as invalid JSON payload, missing mandatory fields, or incorrect data types. If the module is not supported, a 404 would be returned. Authentication errors result in 401. Incorrect URI gives 404. Privilege issues give 403 or 401.
Key principle: Authentication proves identity; authorization controls what that identity can do after login. Both must work for full privileged access.
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 device does not support RESTCONF
Why it's wrong here
If RESTCONF were not supported, the response would be 501 or 404, not 400.
✗
The user does not have sufficient privileges
Why it's wrong here
Insufficient privileges would result in a 401 or 403 error.
✗
The URI is incorrect
Why it's wrong here
An incorrect URI typically yields a 404 Not Found.
✗
The YANG module is not loaded
Why it's wrong here
YANG module not loaded would cause a 404 error, not 400.
✓
The JSON payload contains incorrect data types or missing mandatory leafs
Why this is correct
400 Bad Request indicates a client-side error; invalid payload is a common cause.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Authentication checks who the user is.
Common exam traps
Common exam trap: authentication is not authorization
Logging in proves the user can authenticate. It does not automatically mean the user is allowed to enter privileged or configuration mode. Watch for AAA authorization, privilege level and command authorization details.
Detailed technical explanation
How to think about this question
This kind of question is testing the difference between identity and permission. A user may successfully log in to a router because authentication is working, but still fail to enter configuration mode because authorization is missing, misconfigured or mapped to a lower privilege level.
KKey Concepts to Remember
Authentication checks who the user is.
Authorization controls what the user is allowed to do after login.
Privilege levels affect access to EXEC and configuration commands.
AAA, TACACS+ and RADIUS can separate login success from command access.
TExam Day Tips
→Do not assume successful login means full administrative access.
→Look for words such as cannot enter configuration mode, privilege level, authorization or command access.
→Separate login problems from permission problems before choosing the answer.
Key takeaway
Authentication proves identity; authorization controls what that identity can do after login. Both must work for full privileged access.
Real-world example
How this comes up in practice
A junior network technician can log in to a core router but cannot reach the enable prompt or configuration mode. The AAA server is authenticating the login — but the authorisation policy only grants privilege level 1, not 15. Authentication (who you are) is working; authorisation (what you can do) is not.
What to study next
Got this wrong? Here's your next step.
Review Cisco AAA concepts — authentication, authorization, and accounting. Study privilege levels (0–15), command authorization under TACACS+, and how RADIUS differs. Then practise related 350-501 questions on access control and AAA configuration.
Automation and Assurance — This question tests Automation and Assurance — Authentication checks who the user is..
What is the correct answer to this question?
The correct answer is: The JSON payload contains incorrect data types or missing mandatory leafs — A 400 Bad Request typically indicates a client error, such as invalid JSON payload, missing mandatory fields, or incorrect data types. If the module is not supported, a 404 would be returned. Authentication errors result in 401. Incorrect URI gives 404. Privilege issues give 403 or 401.
What should I do if I get this 350-501 question wrong?
Review Cisco AAA concepts — authentication, authorization, and accounting. Study privilege levels (0–15), command authorization under TACACS+, and how RADIUS differs. Then practise related 350-501 questions on access control and AAA configuration.
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?
Authentication checks who the user is.
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