ANS-C01 Network Management and Operations Practice Question
A company runs a critical application on EC2 instances in an Auto Scaling group across two Availability Zones. The application is fronted by an Application Load Balancer (ALB). The network team recently migrated from a transit VPC to a transit gateway for inter-VPC connectivity. After the migration, users experience intermittent connectivity failures. The team checks the ALB target group and sees that health checks are passing. However, from an EC2 instance in the same VPC, they can reach the ALB but not the application. They notice that the application sends traffic to an internal DNS server that is in a different VPC, and the application depends on that DNS resolution. The transit gateway route tables are configured to propagate routes from attached VPCs. The DNS server is reachable from the application VPC over the transit gateway. What is the MOST likely cause of the intermittent failures?
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 DNS server returns different IP addresses for the same DNS name, and some IPs are not reachable due to route table misconfiguration.
If the DNS server returns different IP addresses for the same DNS name due to DNS round-robin or time-to-live (TTL) caching, the application may get an IP address that is not reachable (e.g., from a different VPC or a terminated instance). The health check may pass because the target group health check uses the ALB's IP, not the application's DNS resolution. Option A is plausible but less likely because the DNS server is reachable. Option B could cause total failure, not intermittent. Option D would affect all traffic, not just intermittent.
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 transit gateway route tables have a blackhole route for the DNS server's VPC.
Why it's wrong here
Would cause persistent failure, not intermittent.
- ✗
The Auto Scaling group is scaling in and out frequently, causing application instances to be terminated during DNS resolution.
Why it's wrong here
Would cause timeouts during scaling events but not intermittent in steady state.
- ✓
The DNS server returns different IP addresses for the same DNS name, and some IPs are not reachable due to route table misconfiguration.
Why this is correct
DNS changes with TTL can cause intermittent reachability to specific IPs.
- ✗
The ALB security group does not allow traffic from the application instances on the ephemeral ports.
Why it's wrong here
Would affect health checks and all traffic, not intermittent.
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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.