ANS-C01 Network Management and Operations Practice Question
A company has a VPC with multiple subnets across Availability Zones. An application load balancer (ALB) is deployed in public subnets. The network team notices that traffic from the ALB to targets in private subnets is intermittently failing. The targets are healthy. What is the MOST likely cause?
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 target security group does not allow traffic from the ALB's security group.
The ALB communicates with targets using its private IP addresses. For traffic to reach the targets, the target security group must allow inbound traffic from the ALB's security group. If this rule is missing, traffic from the ALB will be dropped by the target security group, even though the targets are healthy. Option A is incorrect because the ALB is already deployed in public subnets, so it is associated with a public subnet. Option C is incorrect because network ACLs are stateless and only affect traffic crossing subnet boundaries; if the NACL blocked outbound traffic from targets, there would be issues with response traffic, but the question states traffic from ALB to targets is failing, so the issue is likely inbound to targets. Option D is incorrect because cross-zone load balancing is enabled by default and would not cause intermittent failures; it only affects distribution of traffic across zones.
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 ALB is not associated with a public subnet.
Why it's wrong here
The ALB is already associated with a public subnet. This is not the cause because the ALB is in public subnets and can reach private subnet targets.
- ✓
The target security group does not allow traffic from the ALB's security group.
Why this is correct
Correct. The target security group must allow inbound traffic from the ALB's security group. Without this rule, traffic from the ALB will be blocked.
- ✗
The network ACL for the target subnets blocks outbound traffic.
Why it's wrong here
The network ACL for the target subnets blocks outbound traffic: Incorrect. Network ACLs are stateless; if they blocked outbound traffic from targets, the targets would not be able to send responses, but the failure is on the inbound traffic from ALB to targets. Also, the question states targets are healthy, implying they can send responses. The issue is likely with the security group inbound rule.
- ✗
Cross-zone load balancing is disabled.
Why it's wrong here
Cross-zone load balancing is disabled: Incorrect. Cross-zone load balancing is enabled by default for ALBs and would not cause intermittent failures. It affects how traffic is distributed across zones, not whether traffic reaches targets.
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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.