200-901 Infrastructure and Automation Practice Question
This 200-901 practice question tests your understanding of infrastructure and automation. Read the scenario carefully and evaluate each option against the stated constraints before committing to an answer. 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 developer is using Python requests library to interact with a Cisco IOS XE device's REST API. The call returns a 400 Bad Request status. The payload is correctly formatted JSON. 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 authentication credentials are missing or incorrect in the request header
A 400 Bad Request status from a Cisco IOS XE REST API indicates a client-side error, typically related to malformed syntax or missing required elements. Since the JSON payload is confirmed as correctly formatted, the most likely cause is missing or incorrect authentication credentials in the request header, as the API requires valid credentials (e.g., Basic Auth with username:password encoded in Base64) to process the request. Without proper authentication, the server rejects the request with a 400 status before even evaluating the payload.
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 authentication credentials are missing or incorrect in the request header
Why this is correct
400 Bad Request commonly indicates missing or invalid authentication headers.
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 device's API service is not enabled
Why it's wrong here
If the service is disabled, the request would likely timeout or return a connection refusal.
✗
The requested URL path is incorrect
Why it's wrong here
An incorrect URL would typically result in a 404 Not Found, not 400.
✗
The JSON payload contains a syntax error
Why it's wrong here
The stem says payload is correctly formatted.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the distinction between 400 Bad Request (client-side syntax/header issues) and 401 Unauthorized (invalid credentials), tricking candidates into assuming authentication errors always return 401, when in fact missing or malformed authentication headers can trigger a 400.
Detailed technical explanation
How to think about this question
Cisco IOS XE REST API uses HTTP Basic Authentication by default, where the Authorization header must contain 'Basic ' followed by a Base64-encoded string of 'username:password'. If this header is missing or malformed, the server responds with 400 Bad Request rather than 401 Unauthorized, because the API gateway validates the header structure before checking credentials. In real-world scenarios, developers often forget to set the auth parameter in the requests library (e.g., `requests.get(url, auth=('user', 'pass'))`), leading to this exact error.
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.
Infrastructure and Automation — This question tests Infrastructure and Automation — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The authentication credentials are missing or incorrect in the request header — A 400 Bad Request status from a Cisco IOS XE REST API indicates a client-side error, typically related to malformed syntax or missing required elements. Since the JSON payload is confirmed as correctly formatted, the most likely cause is missing or incorrect authentication credentials in the request header, as the API requires valid credentials (e.g., Basic Auth with username:password encoded in Base64) to process the request. Without proper authentication, the server rejects the request with a 400 status before even evaluating the payload.
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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