- A
Create a Gateway VPC endpoint for S3 and an Interface VPC endpoint for Secrets Manager, and ensure the subnet route tables / endpoint routing directs those service calls to the endpoints instead of the NAT gateway.
This is the most cost-effective change because it removes the need to traverse the NAT gateway for those AWS service calls. S3 uses a Gateway VPC endpoint (route-table-based) for traffic to the S3 prefix list, so requests to S3 stay on the AWS network. Secrets Manager uses an Interface VPC endpoint (ENIs with private DNS), so requests to Secrets Manager stay private within the VPC/VPC endpoint network path. Because the application still calls the same AWS APIs, there is no logic change, and NAT data-processing charges drop to near zero for S3/Secrets Manager traffic.
- B
Keep the NAT gateway, but add AWS WAF rules to block non-service outbound requests to reduce NAT usage.
Why wrong: WAF rules don’t control arbitrary outbound TCP connections initiated from EC2 instances. NAT gateway charges are driven by actual outbound traffic volume (data processed) and active connections, not by WAF policy. Unless the instances themselves stop making those outbound calls, the NAT gateway still processes the traffic and continues to incur charges.
- C
Disable IPv4 on the VPC subnets and rely on IPv6-only egress to reduce NAT gateway costs.
Why wrong: NAT gateways are used specifically for private IPv4 egress to reach the internet or other networks. Switching to IPv6-only does not inherently eliminate the cost drivers for private service connectivity; it also requires a correct egress architecture for reaching AWS services and may introduce other components/changes. This does not directly address the most direct cost lever: using VPC endpoints to avoid NAT for calls to S3 and Secrets Manager.
- D
Replace the NAT gateway with a VPC firewall appliance instance to proxy outbound calls and reduce NAT fees.
Why wrong: Using a self-managed proxy/firewall appliance adds additional compute cost, scaling/availability overhead, and operational burden (patching, upgrades, monitoring). It also does not provide the same purpose-built, service-specific private connectivity model that VPC endpoints provide for S3 and Secrets Manager, so it is unlikely to be more cost-effective than adding the correct endpoints.
Quick Answer
The answer is to create a Gateway VPC Endpoint for S3 and an Interface VPC Endpoint for Secrets Manager, which is the most cost-effective change. This is correct because VPC Endpoints allow private connectivity to AWS services without traversing a NAT gateway, eliminating both the hourly charge and per-GB data processing fees that drive up vpc endpoint vs nat gateway cost. By using route tables to direct traffic to the endpoints instead of the NAT gateway, the application logic remains unchanged—the same DNS names resolve privately within the AWS network. On the SAA-C03 exam, this scenario tests your understanding of when to use Gateway versus Interface endpoints: Gateway endpoints are free and used for S3 and DynamoDB, while Interface endpoints incur hourly costs but are still cheaper than a NAT gateway for services like Secrets Manager. A common trap is assuming all endpoints cost the same or that a NAT gateway is needed for private subnet access. Memory tip: “Gateway for S3 is free, Interface for secrets saves fees.”
SAA-C03 Design Cost-Optimized Architectures Practice Question
This SAA-C03 practice question tests your understanding of design cost-optimized architectures. Match the stated requirement to the specific cloud service, access model, or configuration option — many options are valid in isolation but not for this scenario. 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 hosts an application on EC2 instances in private subnets. The instances must (1) read objects from Amazon S3 and (2) retrieve secrets from AWS Secrets Manager. The team currently sends all outbound traffic through a NAT gateway to reach both services. They want to reduce monthly cost while keeping traffic private (no internet egress) and without changing application logic. Which change is the most cost-effective?
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
Create a Gateway VPC endpoint for S3 and an Interface VPC endpoint for Secrets Manager, and ensure the subnet route tables / endpoint routing directs those service calls to the endpoints instead of the NAT gateway.
Option A is correct because Gateway VPC Endpoints for S3 and Interface VPC Endpoints for Secrets Manager allow private connectivity to these AWS services without traversing the internet or a NAT gateway. This eliminates NAT gateway hourly charges and data processing fees, reducing costs while keeping traffic within the AWS network. The application logic remains unchanged as the endpoints are accessed via the same DNS names, with route tables directing traffic to the endpoints instead of the NAT gateway.
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.
- ✓
Create a Gateway VPC endpoint for S3 and an Interface VPC endpoint for Secrets Manager, and ensure the subnet route tables / endpoint routing directs those service calls to the endpoints instead of the NAT gateway.
Why this is correct
This is the most cost-effective change because it removes the need to traverse the NAT gateway for those AWS service calls. S3 uses a Gateway VPC endpoint (route-table-based) for traffic to the S3 prefix list, so requests to S3 stay on the AWS network. Secrets Manager uses an Interface VPC endpoint (ENIs with private DNS), so requests to Secrets Manager stay private within the VPC/VPC endpoint network path. Because the application still calls the same AWS APIs, there is no logic change, and NAT data-processing charges drop to near zero for S3/Secrets Manager traffic.
Related concept
Read the scenario before looking for a memorised answer.
- ✗
Keep the NAT gateway, but add AWS WAF rules to block non-service outbound requests to reduce NAT usage.
Why it's wrong here
WAF rules don’t control arbitrary outbound TCP connections initiated from EC2 instances. NAT gateway charges are driven by actual outbound traffic volume (data processed) and active connections, not by WAF policy. Unless the instances themselves stop making those outbound calls, the NAT gateway still processes the traffic and continues to incur charges.
- ✗
Disable IPv4 on the VPC subnets and rely on IPv6-only egress to reduce NAT gateway costs.
Why it's wrong here
NAT gateways are used specifically for private IPv4 egress to reach the internet or other networks. Switching to IPv6-only does not inherently eliminate the cost drivers for private service connectivity; it also requires a correct egress architecture for reaching AWS services and may introduce other components/changes. This does not directly address the most direct cost lever: using VPC endpoints to avoid NAT for calls to S3 and Secrets Manager.
- ✗
Replace the NAT gateway with a VPC firewall appliance instance to proxy outbound calls and reduce NAT fees.
Why it's wrong here
Using a self-managed proxy/firewall appliance adds additional compute cost, scaling/availability overhead, and operational burden (patching, upgrades, monitoring). It also does not provide the same purpose-built, service-specific private connectivity model that VPC endpoints provide for S3 and Secrets Manager, so it is unlikely to be more cost-effective than adding the correct endpoints.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates may assume NAT gateways are the only way to provide private subnet internet access, overlooking that VPC endpoints can provide private, cost-effective connectivity to specific AWS services without internet egress.
Detailed technical explanation
How to think about this question
Gateway VPC Endpoints for S3 use prefix lists and route table entries to route traffic directly to S3 without leaving the AWS network, while Interface VPC Endpoints for Secrets Manager use an Elastic Network Interface (ENI) with a private IP address, leveraging AWS PrivateLink. Both endpoints support AWS Private DNS, so existing DNS names (e.g., s3.amazonaws.com) resolve to private IPs, ensuring zero application changes. The cost savings come from eliminating NAT gateway hourly charges ($0.045/hour per AZ) and data processing fees ($0.045/GB), replaced by endpoint hourly charges ($0.01/hour for Gateway, $0.01/hour for Interface) and data processing fees ($0.01/GB for Gateway, $0.01/GB for Interface), which are significantly lower for high-volume traffic.
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 startup's cloud architect reviews their monthly bill and notices costs are higher than expected for a long-running batch job. Switching from on-demand instances to Reserved Instances — or using Spot/Preemptible VMs — can reduce compute costs by up to 72 %. Questions like this test whether you understand the tradeoffs between commitment, flexibility, and cost across cloud pricing models.
What to study next
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FAQ
Questions learners often ask
What does this SAA-C03 question test?
Design Cost-Optimized Architectures — This question tests Design Cost-Optimized Architectures — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Create a Gateway VPC endpoint for S3 and an Interface VPC endpoint for Secrets Manager, and ensure the subnet route tables / endpoint routing directs those service calls to the endpoints instead of the NAT gateway. — Option A is correct because Gateway VPC Endpoints for S3 and Interface VPC Endpoints for Secrets Manager allow private connectivity to these AWS services without traversing the internet or a NAT gateway. This eliminates NAT gateway hourly charges and data processing fees, reducing costs while keeping traffic within the AWS network. The application logic remains unchanged as the endpoints are accessed via the same DNS names, with route tables directing traffic to the endpoints instead of the NAT gateway.
What should I do if I get this SAA-C03 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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Same concept, more angles
3 more ways this is tested on SAA-C03
These questions test the same concept from different angles. Work through them to make sure you can recognise it however the exam phrases it.
Variation 1. A company hosts application servers in private subnets. They must access Amazon S3 and read secrets from AWS Secrets Manager, but they want to avoid internet egress. They currently use a NAT gateway and see high NAT-related costs. What change most directly reduces cost while keeping traffic on the AWS network?
medium- A.Keep the NAT gateway and reduce instance size to lower NAT throughput charges
- ✓ B.Create a Gateway VPC endpoint for S3 and an Interface VPC endpoint for Secrets Manager, then route requests via those endpoints instead of NAT
- C.Switch Secrets Manager to use S3 as a storage backend so both services can use the S3 endpoint only
- D.Move the private subnets to public subnets and attach an Internet Gateway to eliminate NAT
Why B: Option B is correct because it replaces the NAT gateway with VPC endpoints, which are free of data processing charges and keep all traffic within the AWS network. A Gateway VPC endpoint for S3 uses prefix lists and route table entries to send S3 traffic directly over the AWS backbone, while an Interface VPC endpoint for Secrets Manager uses AWS PrivateLink to provide private connectivity without internet egress. This eliminates NAT gateway hourly and data processing costs, directly addressing the cost concern.
Variation 2. A company runs an application on EC2 instances in private subnets. The instances must access Amazon S3, and the team currently routes all outbound traffic to the internet through a NAT Gateway. Monthly NAT Gateway charges increased significantly, even though the application only needs to call S3 (not access other public internet services). Which change will most directly reduce NAT Gateway charges while keeping S3 access working?
medium- ✓ A.Create a gateway VPC endpoint for S3 and update the private route tables so S3 traffic uses the endpoint instead of the NAT Gateway.
- B.Enable S3 Transfer Acceleration on the bucket to reduce the number of S3 calls that go through the NAT Gateway.
- C.Switch the EC2 instances to public subnets so S3 calls can use direct internet routing without NAT.
- D.Increase the NAT Gateway TCP idle timeout so fewer connections are billed separately for S3 traffic.
Why A: A gateway VPC endpoint for S3 allows instances in private subnets to access S3 over the AWS network without traversing the internet. By updating the private route tables to direct S3 traffic to the endpoint, the NAT Gateway is bypassed, eliminating the per-GB data processing charges and hourly NAT Gateway fees for that traffic. This directly reduces costs while maintaining secure, private access to S3.
Variation 3. A data-processing application runs in private subnets and needs to read objects from Amazon S3 and write items to Amazon DynamoDB. The team currently routes all outbound traffic through a NAT Gateway, and monthly networking charges are rising. Which two changes will most directly reduce cost while keeping traffic on the AWS network? Select two.
medium- ✓ A.Add an S3 gateway VPC endpoint to the private route tables.
- ✓ B.Add a DynamoDB gateway VPC endpoint to the private route tables.
- C.Add a second NAT Gateway in another Availability Zone.
- D.Move the instances into public subnets so they can reach AWS services directly.
- E.Send private subnet traffic to S3 and DynamoDB through an Internet Gateway route.
Why A: Adding an S3 gateway VPC endpoint allows instances in private subnets to access S3 directly over the AWS network without traversing the NAT Gateway, eliminating NAT data processing charges for S3 traffic. This reduces monthly networking costs while keeping traffic within the AWS backbone.
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Last reviewed: Jun 11, 2026
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