200-901 Understanding and Using APIs Practice Question
This 200-901 practice question tests your understanding of understanding and using apis. 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.
Exhibit
Refer to the exhibit.
RESTCONF request:
GET /restconf/data/Cisco-IOS-XE-native:native/interface/GigabitEthernet=1/0/1
Headers:
Accept: application/yang-data+json
Response:
{
"Cisco-IOS-XE-native:GigabitEthernet": [{
"name": "1/0/1",
"description": "Connected to core",
"ip": {
"address": {
"primary": {
"address": "192.168.1.1",
"mask": "255.255.255.0"
}
}
}
}]
}
A network engineer attempts to modify the IP address of GigabitEthernet1/0/1 using the Cisco IOS-XE RESTCONF API. They send a PUT request with a modified JSON body but receive 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.
Refer to the exhibit.
RESTCONF request:
GET /restconf/data/Cisco-IOS-XE-native:native/interface/GigabitEthernet=1/0/1
Headers:
Accept: application/yang-data+json
Response:
{
"Cisco-IOS-XE-native:GigabitEthernet": [{
"name": "1/0/1",
"description": "Connected to core",
"ip": {
"address": {
"primary": {
"address": "192.168.1.1",
"mask": "255.255.255.0"
}
}
}
}]
}
A
The Accept header should be application/json.
Why wrong: The Accept header shown is correct for RESTCONF (application/yang-data+json). Changing to application/json may not be supported.
B
The Content-Type header is missing or set incorrectly.
Why wrong: While Content-Type is required, the error here is more likely due to incorrect request body structure; a missing header would cause a different error.
C
The API is not enabled on the device.
Why wrong: If the API were disabled, the response would be 404 Not Found or connection refused, not 400.
D
The request body does not include the full resource hierarchy.
RESTCONF PUT requires the entire data tree for the resource. Omitting parent containers leads to 400.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The request body does not include the full resource hierarchy.
D is correct because RESTCONF requires the request body to contain the full resource hierarchy (e.g., the entire YANG data tree for the interface) when using PUT, as PUT is a full replacement operation. A 400 Bad Request error typically indicates a malformed request, and omitting mandatory parent or sibling nodes in the JSON body violates the YANG schema, causing the server to reject the request.
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 Accept header should be application/json.
Why it's wrong here
The Accept header shown is correct for RESTCONF (application/yang-data+json). Changing to application/json may not be supported.
✗
The Content-Type header is missing or set incorrectly.
Why it's wrong here
While Content-Type is required, the error here is more likely due to incorrect request body structure; a missing header would cause a different error.
✗
The API is not enabled on the device.
Why it's wrong here
If the API were disabled, the response would be 404 Not Found or connection refused, not 400.
✓
The request body does not include the full resource hierarchy.
Why this is correct
RESTCONF PUT requires the entire data tree for the resource. Omitting parent containers leads to 400.
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 PUT (full replacement) and PATCH (partial update) in RESTCONF, and the trap here is that candidates mistakenly think a 400 error is due to missing headers or API availability, rather than recognizing that PUT requires the complete resource hierarchy in the request body.
Trap categories for this question
Command / output trap
The Accept header shown is correct for RESTCONF (application/yang-data+json). Changing to application/json may not be supported.
Detailed technical explanation
How to think about this question
RESTCONF (RFC 8040) mandates that PUT requests replace the entire target resource; the JSON body must conform to the YANG module's data hierarchy, including all mandatory leafs and containers. For example, modifying just the IP address of an interface without including the interface name, type, and enabled status would violate the ietf-interfaces YANG model, triggering a 400 error with a detailed error-tag in the response. In real-world automation, this often catches engineers who assume partial updates are allowed with PUT, whereas PATCH should be used for partial modifications.
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 200-901 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.
Understanding and Using APIs — This question tests Understanding and Using APIs — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The request body does not include the full resource hierarchy. — D is correct because RESTCONF requires the request body to contain the full resource hierarchy (e.g., the entire YANG data tree for the interface) when using PUT, as PUT is a full replacement operation. A 400 Bad Request error typically indicates a malformed request, and omitting mandatory parent or sibling nodes in the JSON body violates the YANG schema, causing the server to reject the request.
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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