The correct answer is that the configuration uses 'netmask' instead of 'prefix-length'. NETCONF relies on YANG models, and the standard ietf-interfaces and ietf-ip YANG models for IPv4 addresses define the IP address using the leaf 'prefix-length', which expects an integer (e.g., 24 for a 255.255.255.0 mask). The 'netmask' field is not a valid leaf in these models, so the NETCONF server will either reject the configuration or apply it incorrectly, leaving the interface without a properly assigned IP address and unable to pass traffic. On the Cisco DevNet Associate 200-901 exam, this tests your understanding of YANG data modeling and NETCONF protocol constraints—a common trap is assuming traditional CLI syntax like 'netmask' works in NETCONF. Remember: YANG speaks in prefix-lengths, not dotted decimals; think "prefix, not mask" to avoid this pitfall.
200-901 Software Development and Design Practice Question
This 200-901 practice question tests your understanding of software development and design. 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.
Refer to the exhibit. An engineer uses NETCONF to configure an interface. After applying this configuration, the interface is administratively up. However, the interface does not pass traffic. 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 IP address configuration uses 'netmask' instead of 'prefix-length'
NETCONF uses YANG models that define IP address configuration with the leaf 'prefix-length' (an integer), not 'netmask'. The 'netmask' field is not a valid leaf in the standard ietf-interfaces or ietf-ip YANG model for IPv4 addresses, so the NETCONF server will either reject the configuration or apply it incorrectly, leaving the interface without a proper IP address and unable to pass traffic.
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 interface type is misspelled
Why it's wrong here
The type is valid.
✗
The 'enabled' field should be a string instead of a boolean
Why it's wrong here
Boolean is correct for JSON encoding of YANG boolean.
✓
The IP address configuration uses 'netmask' instead of 'prefix-length'
Why this is correct
The ietf-ip model requires prefix-length, not netmask.
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 interface name is invalid
Why it's wrong here
The name is valid.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the distinction between CLI-style 'netmask' and YANG-model 'prefix-length' in NETCONF/RESTCONF questions, trapping candidates who assume the old CLI syntax works in model-driven APIs.
Detailed technical explanation
How to think about this question
In the ietf-ip YANG model (RFC 7277), IPv4 addresses are modeled under 'ipv4/address' with a list containing 'ip' (the address) and 'prefix-length' (an integer 0-32). The 'netmask' is a legacy concept from CLI and is not part of the YANG model; NETCONF requires prefix-length. When using 'netmask', the NETCONF server may silently ignore the field or treat it as an unknown node, resulting in no IP address being assigned, which prevents traffic forwarding even if the interface is up.
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 network engineer segments a warehouse floor into three subnets: 20 scanners, 5 printers, and 2 management hosts. Picking the wrong mask wastes addresses or leaves too few usable hosts. Exam questions test whether you can apply CIDR notation, calculate block size, and identify the correct usable-host range for a given prefix.
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.
Software Development and Design — This question tests Software Development and Design — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The IP address configuration uses 'netmask' instead of 'prefix-length' — NETCONF uses YANG models that define IP address configuration with the leaf 'prefix-length' (an integer), not 'netmask'. The 'netmask' field is not a valid leaf in the standard ietf-interfaces or ietf-ip YANG model for IPv4 addresses, so the NETCONF server will either reject the configuration or apply it incorrectly, leaving the interface without a proper IP address and unable to pass traffic.
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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