The answer is the SourceDestCheck flag being enabled, which prevents the Lambda function from sending traffic it is not the source or destination of. This setting, enabled by default on Elastic Network Interfaces (ENIs), causes the VPC to drop any packets where the ENI’s IP is neither the source nor the destination IP address. In the context of the AWS Certified Advanced Networking Specialty ANS-C01 exam, this scenario tests your understanding of how Lambda functions attached to a VPC use ENIs, and how the source/destination check can silently break traffic forwarding—such as when a Lambda acts as a NAT or intermediary to an on-premises server over Direct Connect. A common trap is assuming security groups or subnet routing are the root cause, but the exhibit’s missing rules point directly to the ENI’s default behavior. Memory tip: think “Lambda as a middleman needs the check off—if it’s not the source or destination, the packet gets tossed.”
ANS-C01 Network Implementation Practice Question
This ANS-C01 practice question tests your understanding of network implementation. 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.
Network Topology
Refer to the exhibit. A Lambda function is attached to a VPC using the network interface eni-1234567890abcdef0. The Lambda function needs to send traffic to an on-premises server via a Direct Connect connection. The traffic is failing. Which setting on the network interface is most likely causing the issue?
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 SourceDestCheck flag is enabled, which prevents the Lambda function from sending traffic that it is not the source or destination of.
The SourceDestCheck flag is set to true (default). For Lambda functions that are not the source or destination of traffic (e.g., when acting as a NAT or forwarding traffic), this check must be disabled. In this scenario, if the Lambda function is forwarding traffic, the source/destination check would drop packets. Option A (multiple IPs) is not an issue; Option B (security group) could be but the exhibit doesn't show rules; Option C (subnet) is not the direct cause.
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.
✗
The network interface is in a subnet that does not have a route to the Direct Connect.
Why it's wrong here
The route table is not shown, but the exhibit does not indicate a routing issue.
✗
The network interface has two private IP addresses.
Why it's wrong here
Multiple IP addresses are allowed and do not cause traffic failure.
✓
The SourceDestCheck flag is enabled, which prevents the Lambda function from sending traffic that it is not the source or destination of.
Why this is correct
Lambda functions that forward traffic need to disable source/destination check.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
The security group attached to the network interface may be blocking outbound traffic.
Why it's wrong here
Possible but not indicated; the SourceDestCheck is a more direct cause for forwarding scenarios.
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.
Trap categories for this question
Command / output trap
The route table is not shown, but the exhibit does not indicate a routing issue.
Scenario analysis trap
Possible but not indicated; the SourceDestCheck is a more direct cause for forwarding scenarios.
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 healthcare organisation deploys an application with a public-facing web tier and a private database tier. The database subnet has no public IP and only accepts connections from the web tier's security group. Questions like this test whether you can design cloud network isolation using VNets/VPCs, subnets, and security group rules.
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 ANS-C01 subnetting questions on CIDR, address ranges, and subnet selection.
Network Implementation — This question tests Network Implementation — CIDR notation defines the prefix length..
What is the correct answer to this question?
The correct answer is: The SourceDestCheck flag is enabled, which prevents the Lambda function from sending traffic that it is not the source or destination of. — The SourceDestCheck flag is set to true (default). For Lambda functions that are not the source or destination of traffic (e.g., when acting as a NAT or forwarding traffic), this check must be disabled. In this scenario, if the Lambda function is forwarding traffic, the source/destination check would drop packets. Option A (multiple IPs) is not an issue; Option B (security group) could be but the exhibit doesn't show rules; Option C (subnet) is not the direct cause.
What should I do if I get this ANS-C01 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 ANS-C01 subnetting questions on CIDR, address ranges, and subnet selection.
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?
CIDR notation defines the prefix length.
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