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 creating a YANG data model for a new interface feature. The model must allow the user to choose from a predefined set of values for the 'duplex' leaf. Which YANG statement should be used to restrict the values to 'full', 'half', and 'auto'?
choice duplex-options { case full; case half; case auto; }
Why wrong: Choice is for selecting between child nodes, not for leaf values.
type string;
Why wrong: String allows any value.
type leafref { path '/other:duplex-list'; }
Why wrong: Leafref references an existing leaf.
type enumeration { enum full; enum half; enum auto; }
Enumeration restricts to listed values.
Answer choices
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
type enumeration { enum full; enum half; enum auto; }
Option D is correct because the 'type enumeration' statement in YANG defines a leaf that can only take one of the explicitly listed enum values. By specifying 'enum full;', 'enum half;', and 'enum auto;', the developer restricts the 'duplex' leaf to exactly those three predefined strings, which matches the requirement.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
For each option: why learners choose it and why it is or isn't the right answer here.
choice duplex-options { case full; case half; case auto; }
Why it's wrong here
Choice is for selecting between child nodes, not for leaf values.
type string;
Why it's wrong here
String allows any value.
type leafref { path '/other:duplex-list'; }
Why it's wrong here
Leafref references an existing leaf.
type enumeration { enum full; enum half; enum auto; }
Why this is correct
Enumeration restricts to listed values.
Related concept
Read the scenario before looking for a memorised answer.
Common exam traps
Cisco often tests the distinction between YANG's 'choice' statement (which selects among different schema branches) and the 'enumeration' type (which restricts a single leaf's value), leading candidates to mistakenly choose 'choice' when they need a value restriction.
Detailed technical explanation
YANG's 'enumeration' built-in type is defined in RFC 7950 and maps to a set of named integer values (starting at 0 by default). When a leaf is of type enumeration, the NETCONF or RESTCONF server validates that the input string exactly matches one of the enum names, making it ideal for configuration parameters like duplex mode. In real-world device YANG models (e.g., Cisco IOS-XE native models), 'duplex' is commonly modeled as an enumeration with values 'auto', 'full', and 'half'.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
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.
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FAQ
Infrastructure and Automation — This question tests Infrastructure and Automation — Read the scenario before looking for a memorised answer..
The correct answer is: type enumeration { enum full; enum half; enum auto; } — Option D is correct because the 'type enumeration' statement in YANG defines a leaf that can only take one of the explicitly listed enum values. By specifying 'enum full;', 'enum half;', and 'enum auto;', the developer restricts the 'duplex' leaf to exactly those three predefined strings, which matches the requirement.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Read the scenario before looking for a memorised answer.
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Last reviewed: Jun 24, 2026
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