ANS-C01 Network Design Practice Question
A company runs a multi-tier web application in a VPC with public and private subnets across two Availability Zones. The web tier uses an Application Load Balancer (ALB) in the public subnets, and the application tier uses EC2 instances in private subnets. The database tier uses an RDS MySQL Multi-AZ instance in private subnets. The company has implemented a network ACL (NACL) on the private subnets to allow only traffic from the ALB security group. Recently, the application tier instances are unable to connect to the RDS database. The security group for RDS allows inbound traffic on port 3306 from the application tier security group. The network team has verified that the application tier instances can reach the internet through a NAT Gateway. What is the MOST likely cause of the connectivity issue?
⚠ Common exam trap
Many exam-takers assume NACLs are stateful like security groups, leading them to overlook the need for explicit outbound rules for traffic initiated from within the subnet.
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
✓
The NACL on the private subnets is blocking outbound traffic from the application tier to the RDS database.
The NACL on the private subnets is stateful only for inbound rules; outbound traffic is evaluated separately. Since the NACL allows only inbound traffic from the ALB security group but does not have an outbound rule permitting traffic from the application tier to the RDS database (port 3306), the outbound SYN packets from the application instances are dropped, preventing the TCP handshake from completing.
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 NACL on the private subnets is blocking outbound traffic from the application tier to the RDS database.
Why this is correct
Correct: NACL's stateless nature requires explicit outbound rules for ephemeral ports.
- ✗
The application tier instances are in a different Availability Zone than the RDS primary instance.
Why it's wrong here
Multi-AZ RDS is accessible from any AZ.
- ✗
The NAT Gateway is blocking traffic to the RDS endpoint.
Why it's wrong here
NAT is for internet; RDS is within VPC.
- ✗
The RDS security group is not allowing inbound traffic from the application tier security group.
Why it's wrong here
The security group rule is correct.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This ANS-C01 practice question is part of Courseiva's free Amazon Web Services certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the ANS-C01 exam.