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.
Exhibit
HTTP/1.1 400 Bad Request
Content-Type: application/yang-data+json
{
"ietf-restconf:errors": {
"error": [
{
"error-tag": "malformed-message",
"error-message": "Invalid JSON syntax at line 3, column 12"
}
]
}
}
Refer to the exhibit. A developer receives this response when attempting to send a PATCH request to modify a YANG data node via RESTCONF. 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 is malformed
A PATCH request to modify a YANG data node via RESTCONF returns a 400 Bad Request status when the JSON payload is malformed. RESTCONF (RFC 8040) requires the request body to conform to the YANG module's data model; if the JSON syntax is invalid or the data does not match the schema (e.g., missing required fields, incorrect data types), the server rejects the request with a 400 error. The 400 status code specifically indicates a client-side error in the request payload, not an authentication or resource existence issue.
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 resource does not exist
Why it's wrong here
Non-existent resource would return 404.
✗
The authentication token is expired
Why it's wrong here
Expired token returns 401.
✗
The YANG model is not supported
Why it's wrong here
Unsupported model would return a different error.
✓
The JSON payload is malformed
Why this is correct
The error-tag 'malformed-message' indicates JSON syntax error.
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.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the distinction between HTTP status codes for RESTCONF errors, and the trap here is that candidates confuse a 400 Bad Request (payload issue) with a 404 Not Found (resource missing) or a 401 Unauthorized (auth issue), especially when the question describes a 'modify' operation that might imply the resource exists.
Detailed technical explanation
How to think about this question
RESTCONF uses HTTP methods to manipulate YANG-defined data stores; a PATCH request must include a JSON or XML payload that strictly follows the YANG schema, including correct data types, mandatory leafs, and proper nesting. The server validates the payload against the YANG module's constraints before applying the change; a malformed payload (e.g., invalid JSON syntax, wrong enum value, or missing mandatory node) triggers a 400 with an error message detailing the validation failure. In real-world scenarios, developers often encounter this when they forget to include a required 'name' field or use an incorrect data type like a string instead of an integer.
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 small business has 20 workstations on the 192.168.1.0/24 network and one public IP from its ISP. The router uses PAT (NAT overload) so all 20 devices share one public address using different source ports. NAT questions test whether you understand the four address terms and which direction each translation applies.
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 JSON payload is malformed — A PATCH request to modify a YANG data node via RESTCONF returns a 400 Bad Request status when the JSON payload is malformed. RESTCONF (RFC 8040) requires the request body to conform to the YANG module's data model; if the JSON syntax is invalid or the data does not match the schema (e.g., missing required fields, incorrect data types), the server rejects the request with a 400 error. The 400 status code specifically indicates a client-side error in the request payload, not an authentication or resource existence issue.
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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