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.
Network Topology
A
The network interface is in a subnet that does not have a route to the Direct Connect.
Why wrong: The route table is not shown, but the exhibit does not indicate a routing issue.
B
The network interface has two private IP addresses.
Why wrong: Multiple IP addresses are allowed and do not cause traffic failure.
C
The SourceDestCheck flag is enabled, which prevents the Lambda function from sending traffic that it is not the source or destination of.
Lambda functions that forward traffic need to disable source/destination check.
D
The security group attached to the network interface may be blocking outbound traffic.
Why wrong: Possible but not indicated; the SourceDestCheck is a more direct cause for forwarding scenarios.
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.
Lambda functions attached to a VPC use elastic network interfaces (ENIs) with the Source/Destination Check flag enabled by default. This flag causes the ENI to drop any traffic where the ENI's IP address is not the source or destination of the packet. Since the Lambda function is sending traffic to an on-premises server via Direct Connect, the ENI is acting as an intermediate hop, and the flag must be disabled for the traffic to pass.
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 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.
Related concept
Read the scenario before looking for a memorised 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: answer the scenario, not the keyword
The ANS-C01 exam often tests the Source/Destination Check flag as a hidden default setting that candidates forget to disable when an ENI is used for forwarding traffic, leading them to incorrectly blame routing or security groups.
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
The Source/Destination Check flag is a feature of AWS VPC ENIs that enforces the host to only process packets where its own IP is the source or destination. This is critical for NAT instances, load balancers, or Lambda functions that need to forward traffic. Disabling this flag allows the ENI to act as a transparent intermediary, forwarding packets between the VPC and on-premises networks over Direct Connect. In practice, this flag is often overlooked when configuring Lambda in a VPC for outbound-only access to on-premises resources.
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 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.
Quick reference
Cloud Service Model Comparison
Model
You Manage
Provider Manages
Examples
IaaS
OS, runtime, apps, data
Hardware, hypervisor, networking
EC2, Azure VMs, GCP Compute Engine
PaaS
Apps and data
OS, runtime, middleware, hardware
Elastic Beanstalk, Azure App Service
SaaS
Data and settings only
Everything else
Microsoft 365, Salesforce, Workday
FaaS / Serverless
Function code only
Infra, scaling, runtime
Lambda, Azure Functions, Cloud Run
CaaS
Containers and apps
Kubernetes, OS, hardware
EKS, AKS, GKE
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 Implementation — This question tests Network Implementation — Read the scenario before looking for a memorised answer..
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. — Lambda functions attached to a VPC use elastic network interfaces (ENIs) with the Source/Destination Check flag enabled by default. This flag causes the ENI to drop any traffic where the ENI's IP address is not the source or destination of the packet. Since the Lambda function is sending traffic to an on-premises server via Direct Connect, the ENI is acting as an intermediate hop, and the flag must be disabled for the traffic to pass.
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.
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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