The answer lies in two configuration changes: ensuring the RDS database’s security group allows inbound traffic from the EC2 instance’s security group on port 3306, and verifying that custom route tables associated with both subnets include the local route for the VPC CIDR 10.0.0.0/16. This is because security groups act as virtual firewalls at the instance level—by default, RDS security groups block all inbound traffic, so explicit permission for the EC2’s security group is required for MySQL connectivity. Meanwhile, even with permissive network ACLs, if custom route tables replace the default main route table, the automatic local route for intra-VPC communication may be missing, preventing traffic from subnet A to subnet B. On the AWS Certified Advanced Networking Specialty ANS-C01 exam, this scenario tests your understanding of how security groups and route tables interact across subnets, a common trap where candidates overlook custom route tables because they assume the default local route always applies. Remember the mnemonic: “SG for ports, RT for paths”—security groups control port access, while route tables ensure the network path exists.
ANS-C01 Network Design Practice Question
This ANS-C01 practice question tests your understanding of network design. 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 has a VPC with CIDR 10.0.0.0/16. They have two subnets: subnet A (10.0.1.0/24) and subnet B (10.0.2.0/24). An EC2 instance in subnet A needs to communicate with an RDS database in subnet B. Both subnets have network ACLs that allow all inbound and outbound traffic. However, the instance cannot connect to the database. Which TWO configuration changes could solve this issue? (Choose TWO.)
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Ensure that the route tables associated with both subnets have a local route for the VPC CIDR.
Option B is correct because the local route for the VPC CIDR (10.0.0.0/16) is automatically added to the main route table, but if custom route tables are associated with subnets A and B, they must explicitly include this local route to enable routing between subnets within the same VPC. Without it, traffic from the EC2 instance in subnet A cannot reach the RDS database in subnet B, even if network ACLs allow all traffic.
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.
✗
Attach an Internet Gateway to the VPC.
Why it's wrong here
Not needed for internal communication.
✓
Ensure that the route tables associated with both subnets have a local route for the VPC CIDR.
Why this is correct
Local route is needed for intra-VPC communication.
Related concept
Read the scenario before looking for a memorised answer.
✗
Create a VPC peering connection between the subnets.
Why it's wrong here
VPC peering is for different VPCs, not within same VPC.
✓
Modify the security group for the RDS database to allow inbound traffic from the EC2 instance's security group on port 3306.
Read the scenario before looking for a memorised answer.
✗
Add a NAT Gateway in a public subnet and route traffic through it.
Why it's wrong here
Unnecessary for internal VPC communication.
Common exam traps
Common exam trap: answer the scenario, not the keyword
AWS often tests the misconception that network ACLs alone control all traffic flow, but candidates forget that security groups must also allow inbound traffic, and that route tables must have a local route for intra-VPC communication.
Detailed technical explanation
How to think about this question
Under the hood, the VPC router uses the route tables to determine where to forward traffic; if the local route is missing, the router has no entry for the destination subnet and drops the packet. Security groups act as a virtual firewall at the instance level, and by default, they deny all inbound traffic; for the RDS database to accept connections from the EC2 instance, the security group must explicitly allow inbound traffic on port 3306 (MySQL/Aurora default) from the EC2 instance's security group or its private IP. In real-world scenarios, misconfigured route tables are a common cause of intra-VPC connectivity failures, especially when custom route tables are created without the local route.
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 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.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Network Design — This question tests Network Design — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Ensure that the route tables associated with both subnets have a local route for the VPC CIDR. — Option B is correct because the local route for the VPC CIDR (10.0.0.0/16) is automatically added to the main route table, but if custom route tables are associated with subnets A and B, they must explicitly include this local route to enable routing between subnets within the same VPC. Without it, traffic from the EC2 instance in subnet A cannot reach the RDS database in subnet B, even if network ACLs allow all traffic.
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.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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