ANS-C01 Network Implementation Practice Question
A company deployed an Application Load Balancer (ALB) in front of a fleet of EC2 instances. Users report intermittent timeouts. The ALB's target group health checks are failing for some instances. The instances are in private subnets with a single NAT Gateway. What is the most likely cause?
⚠ Common exam trap
The ANS-C01 exam often tests the misconception that NAT Gateway configuration affects internal VPC traffic, leading candidates to incorrectly select route table or NAT Gateway issues when the actual problem is security group misconfiguration for ALB health checks.
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 security group on the EC2 instances does not allow inbound traffic from the ALB's security group.
The most likely cause is that the security group on the EC2 instances does not allow inbound traffic from the ALB's security group. ALB health checks originate from the ALB's private IP addresses, and if the instance security group lacks an inbound rule permitting traffic from the ALB's security group (or its CIDR) on the health check port, the health checks will fail, causing intermittent timeouts as the ALB stops routing traffic to unhealthy targets.
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 security group on the EC2 instances does not allow inbound traffic from the ALB's security group.
Why this is correct
ALB health checks originate from the ALB's security group; if not allowed, health checks fail.
- ✗
The route tables in the private subnets do not have a default route pointing to the NAT Gateway.
Why it's wrong here
Missing default route affects outbound internet traffic, not health checks from ALB.
- ✗
The EC2 instances are of an unsupported instance type for ALB target groups.
Why it's wrong here
ALB supports all current EC2 instance types.
- ✗
The NAT Gateway is in a single Availability Zone, causing traffic to fail during an AZ outage.
Why it's wrong here
Health checks do not traverse the NAT Gateway; they are sent directly from the ALB.
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JA
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.