- A
Create a VPC peering connection between the subnets.
Why wrong: VPC peering connects entire VPCs, not subnets.
- B
Configure security groups for the EC2 instances that only allow outbound traffic to the RDS security group, and RDS security group allows inbound from the EC2 security group.
Security groups provide instance-level granularity and stateful filtering.
- C
Assign the same security group to both the EC2 instances and the RDS database.
Why wrong: Same security group allows all traffic between them, but also to others.
- D
Use network ACLs with deny rules for all traffic except between the two subnets.
Why wrong: NACLs are stateless and apply to entire subnets, not specific instances.
Question 1,204 of 1,748
SCS-C02 Infrastructure Security Practice Question
This SCS-C02 practice question tests your understanding of infrastructure security. 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 security engineer is designing a network segmentation strategy for a VPC that hosts sensitive data. The engineer needs to ensure that EC2 instances in a private subnet can communicate with an RDS database in a different private subnet, but cannot communicate with any other resources in the same VPC. Which configuration should be used?
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
✓
Configure security groups for the EC2 instances that only allow outbound traffic to the RDS security group, and RDS security group allows inbound from the EC2 security group.
Security groups act as a virtual firewall at the instance level, allowing stateful traffic filtering. By configuring the EC2 security group to allow outbound traffic only to the RDS security group (using the security group ID as the destination), and the RDS security group to allow inbound traffic only from the EC2 security group, you create a precise, bidirectional allowlist. This ensures that EC2 instances can communicate exclusively with the RDS database, blocking all other traffic within the VPC without relying on IP addresses or subnet CIDRs.
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 VPC peering connection between the subnets.
Why it's wrong here
VPC peering connects entire VPCs, not subnets.
- ✓
Configure security groups for the EC2 instances that only allow outbound traffic to the RDS security group, and RDS security group allows inbound from the EC2 security group.
Why this is correct
Security groups provide instance-level granularity and stateful filtering.
Related concept
Read the scenario before looking for a memorised answer.
- ✗
Assign the same security group to both the EC2 instances and the RDS database.
Why it's wrong here
Same security group allows all traffic between them, but also to others.
- ✗
Use network ACLs with deny rules for all traffic except between the two subnets.
Why it's wrong here
NACLs are stateless and apply to entire subnets, not specific instances.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often confuse security groups with network ACLs or assume that VPC peering or shared security groups are sufficient for fine-grained segmentation, overlooking that security group referencing provides the precise, resource-level isolation required for this scenario.
Detailed technical explanation
How to think about this question
Security group rules support referencing other security groups as sources or destinations, enabling micro-segmentation without IP address dependencies. This is particularly useful in dynamic environments where instance IPs change (e.g., auto-scaling groups). Under the hood, AWS evaluates security group rules using a connection-tracking mechanism (stateful firewall), automatically allowing return traffic for permitted outbound flows, which simplifies rule management compared to stateless NACLs.
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.
Visual reference
What to study next
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FAQ
Questions learners often ask
What does this SCS-C02 question test?
Infrastructure Security — This question tests Infrastructure Security — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Configure security groups for the EC2 instances that only allow outbound traffic to the RDS security group, and RDS security group allows inbound from the EC2 security group. — Security groups act as a virtual firewall at the instance level, allowing stateful traffic filtering. By configuring the EC2 security group to allow outbound traffic only to the RDS security group (using the security group ID as the destination), and the RDS security group to allow inbound traffic only from the EC2 security group, you create a precise, bidirectional allowlist. This ensures that EC2 instances can communicate exclusively with the RDS database, blocking all other traffic within the VPC without relying on IP addresses or subnet CIDRs.
What should I do if I get this SCS-C02 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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Last reviewed: Jul 4, 2026
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