The correct answer is to update the Lambda function configuration to remove the VPC settings, which forces the Lambda service to delete the associated elastic network interface (ENI), and then delete the subnet. This resolves the CloudFormation subnet deletion failure due to Lambda ENI because the ENI is a managed resource created by Lambda when the function is attached to a VPC; you cannot manually delete it through the EC2 console or API. On the AWS Certified Solutions Architect Professional SAP-C02 exam, this scenario tests your understanding of resource dependencies and lifecycle management in CloudFormation, often appearing as a trap where candidates mistakenly try to force-delete the subnet or manually remove the ENI. A key memory tip: Lambda ENIs are “service-owned” — you must break the attachment by updating the function, not by touching the ENI directly.
SAP-C02 Continuous Improvement for Existing Solutions Practice Question
This SAP-C02 practice question tests your understanding of continuous improvement for existing solutions. 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 company uses AWS CloudFormation to manage infrastructure. The stack fails to update with the error: 'Resource handler returned message: The subnet 'subnet-xxx' is in use by a network interface.' The subnet is associated with a Lambda function in a VPC. The CloudFormation template is trying to delete the subnet. What should the company do to resolve this?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Update the Lambda function configuration to remove the VPC settings, then delete the subnet.
Option C is correct because the Lambda function's ENI must be deleted first by deleting the Lambda function or updating its configuration to not use the subnet. Option A is incorrect because the ENI is managed by Lambda and cannot be manually deleted. Option B is incorrect because the subnet is in use, so the stack cannot continue. Option D is incorrect because force delete is not a valid operation for subnets.
Key principle: Count usable hosts — not total addresses — and remember that the network and broadcast addresses are not available to hosts in standard IPv4 subnets.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
✓
Update the Lambda function configuration to remove the VPC settings, then delete the subnet.
Modify the CloudFormation template to ignore the deletion failure using a DeletionPolicy attribute.
Why it's wrong here
DeletionPolicy can retain resources but does not resolve the dependency.
✗
Use the AWS CLI to force delete the subnet.
Why it's wrong here
Subnet cannot be force deleted if in use.
✗
Manually delete the Elastic Network Interface (ENI) from the AWS Management Console.
Why it's wrong here
ENI is managed by Lambda and cannot be manually deleted while in use.
Common exam traps
Common exam trap: usable hosts are not the same as total addresses
Subnetting questions often tempt you into counting all addresses. In normal IPv4 subnets, the network and broadcast addresses are not usable host addresses.
Detailed technical explanation
How to think about this question
Subnetting questions test whether you can identify the network, broadcast address, usable range, mask and correct subnet. Slow down enough to calculate the block size correctly.
KKey Concepts to Remember
CIDR notation defines the prefix length.
Block size helps identify subnet boundaries.
Network and broadcast addresses are not usable hosts in normal IPv4 subnets.
The required host count determines the smallest suitable subnet.
TExam Day Tips
→Write the block size before choosing the subnet.
→Check whether the question asks for hosts, subnets or a specific address range.
→Do not confuse /24, /25, /26 and /27 host counts.
Key takeaway
Count usable hosts — not total addresses — and remember that the network and broadcast addresses are not available to hosts in standard IPv4 subnets.
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.
Review block sizes, usable host formulas (2^n − 2), and how to find network and broadcast addresses for /24 through /30. Then practise related SAP-C02 subnetting questions on CIDR, address ranges, and subnet selection.
Continuous Improvement for Existing Solutions — This question tests Continuous Improvement for Existing Solutions — CIDR notation defines the prefix length..
What is the correct answer to this question?
The correct answer is: Update the Lambda function configuration to remove the VPC settings, then delete the subnet. — Option C is correct because the Lambda function's ENI must be deleted first by deleting the Lambda function or updating its configuration to not use the subnet. Option A is incorrect because the ENI is managed by Lambda and cannot be manually deleted. Option B is incorrect because the subnet is in use, so the stack cannot continue. Option D is incorrect because force delete is not a valid operation for subnets.
What should I do if I get this SAP-C02 question wrong?
Review block sizes, usable host formulas (2^n − 2), and how to find network and broadcast addresses for /24 through /30. Then practise related SAP-C02 subnetting questions on CIDR, address ranges, and subnet selection.
What is the key concept behind this question?
CIDR notation defines the prefix length.
About these practice questions
Courseiva creates original exam-style practice questions with explanations and wrong-answer analysis. It does not publish real exam questions, exam dumps, or protected exam content. Learn why practice questions differ from exam dumps →
Share a tip, memory trick, or ask about the reasoning behind this question. Do not post real exam questions, leaked content, braindumps, or copyrighted exam material. Comments are moderated and may be removed without notice.
This SAP-C02 practice question is part of Courseiva's free Amazon Web Services certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the SAP-C02 exam.
Question Discussion
Share a tip, memory trick, or ask about the reasoning behind this question. Do not post real exam questions, leaked content, braindumps, or copyrighted exam material. Comments are moderated and may be removed without notice.
Sign in to join the discussion.