The answer is that the Lambda function fails because the IAM policy is missing the ec2:CreateNetworkInterfacePermission action. Even if the explicit denial of ec2:CreateNetworkInterface were removed, Lambda requires this specific permission to authorize the service to create and attach an Elastic Network Interface (ENI) in your VPC on your behalf, particularly in cross-account or service-linked scenarios. On the AWS Certified Advanced Networking Specialty ANS-C01 exam, this tests your understanding of the nuanced permissions chain for VPC-enabled Lambda functions, where a common trap is assuming that ec2:CreateNetworkInterface alone is sufficient. The exam often pairs this with a policy that denies or omits the permission action, forcing you to recognize that Lambda acts as a service principal needing explicit consent. A reliable memory tip: think of it as needing both the “create” key and the “permission” lock—without the permission action, the ENI door stays shut.
ANS-C01 Network Design Practice Question
This ANS-C01 practice question tests your understanding of network design. 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.
A network engineer is troubleshooting an issue where an AWS Lambda function cannot create an Elastic Network Interface (ENI) in a VPC. The function has the IAM policy shown in the exhibit. Which statement explains why the function is failing?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The policy is missing the ec2:CreateNetworkInterfacePermission action
The correct answer is D because creating an Elastic Network Interface (ENI) in a VPC requires the `ec2:CreateNetworkInterfacePermission` action in addition to `ec2:CreateNetworkInterface`. The IAM policy shown in the exhibit explicitly denies `ec2:CreateNetworkInterface`, but even if that denial were removed, the Lambda function would still fail without the permission action, as it is necessary for cross-account or service-linked ENI creation scenarios. AWS Lambda uses this permission to allow the Lambda service to attach the ENI to the VPC on your behalf.
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 policy denies the ec2:CreateVpc action which is required to create an ENI
Why it's wrong here
Creating an ENI does not require the ec2:CreateVpc action, as the VPC already exists.
✗
The policy allows ec2:DescribeInstances which conflicts with ENI creation
Why it's wrong here
DescribeInstances is unrelated and does not cause conflicts.
✗
The policy denies the ec2:CreateNetworkInterface action
Why it's wrong here
The policy allows ec2:CreateNetworkInterface, so it is not denied.
✓
The policy is missing the ec2:CreateNetworkInterfacePermission action
Why this is correct
Lambda requires ec2:CreateNetworkInterfacePermission to create ENIs on behalf of the function; without it, the call fails.
Related concept
Read the scenario before looking for a memorised answer.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates focus on the explicit denial of `ec2:CreateNetworkInterface` in the policy and overlook the separate requirement for `ec2:CreateNetworkInterfacePermission`, which is necessary for Lambda to delegate ENI management to the AWS service.
Detailed technical explanation
How to think about this question
When a Lambda function is configured to run in a VPC, it requires an ENI to communicate within the VPC. The Lambda service uses the function's execution role to call `ec2:CreateNetworkInterface` and `ec2:CreateNetworkInterfacePermission` (the latter to grant the Lambda service principal permission to manage the ENI). Without `ec2:CreateNetworkInterfacePermission`, the Lambda service cannot attach the ENI to the VPC hyperplane, leading to a failure even if `ec2:CreateNetworkInterface` is allowed. This is a common misconfiguration because the permission action is often overlooked in favor of the create action.
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 company's IT admin needs to give a contractor read-only access to production logs without sharing account credentials. Using role-based access control (RBAC) and temporary scoped permissions — not a permanent shared password — is the correct pattern. Questions like this test whether you can apply least-privilege access across cloud identity services.
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.
Network Design — This question tests Network Design — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The policy is missing the ec2:CreateNetworkInterfacePermission action — The correct answer is D because creating an Elastic Network Interface (ENI) in a VPC requires the `ec2:CreateNetworkInterfacePermission` action in addition to `ec2:CreateNetworkInterface`. The IAM policy shown in the exhibit explicitly denies `ec2:CreateNetworkInterface`, but even if that denial were removed, the Lambda function would still fail without the permission action, as it is necessary for cross-account or service-linked ENI creation scenarios. AWS Lambda uses this permission to allow the Lambda service to attach the ENI to the VPC on your behalf.
What should I do if I get this ANS-C01 question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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