ANS-C01 Network Implementation Practice Question
A company is using a Network Load Balancer (NLB) to distribute traffic to a fleet of EC2 instances. The NLB is configured with a target group that has health checks enabled. Some instances are marked as unhealthy even though they are running and responding to requests on the health check port. What is a likely cause?
⚠ Common exam trap
The trap here is that candidates often focus on internet-facing connectivity (IGW, route tables) or network ACLs, but the most direct and common cause for health check failures with NLB is the security group on the target instances not allowing traffic from the NLB's 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 security group for the EC2 instances does not allow inbound traffic from the VPC CIDR on the health check port.
When a Network Load Balancer performs health checks, it sends requests from its private IP addresses within the NLB subnet to the target instances. The security group on the EC2 instances must explicitly allow inbound traffic from the VPC CIDR (or the NLB subnet CIDR) on the health check port. If the security group only allows traffic from the internet or a different source, the health check packets are dropped, causing the instances to be marked unhealthy even though they are running and responding to other traffic.
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 route table for the NLB subnet does not have a route to the internet.
Why it's wrong here
Health checks are internal; no internet route needed.
- ✓
The security group for the EC2 instances does not allow inbound traffic from the VPC CIDR on the health check port.
Why this is correct
NLB health checks come from the VPC CIDR; security group must allow that traffic.
- ✗
The internet gateway is not attached to the VPC.
Why it's wrong here
NLB health checks do not use internet gateway.
- ✗
The health check is being blocked by a network ACL that allows inbound traffic only from the internet.
Why it's wrong here
Health checks originate from within the VPC, not internet.
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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.