ANS-C01 Network Design Practice Question
A large e-commerce company operates a multi-tier application across multiple AWS accounts. The web tier is in a VPC (10.0.0.0/16) in Account A, and the application tier is in a separate VPC (10.1.0.0/16) in Account B. Both VPCs are connected via a VPC peering connection. The application tier uses an NLB to distribute traffic to EC2 instances in private subnets. The web tier sends traffic to the NLB's private IP address. Recently, the company migrated the application tier to use AWS PrivateLink instead of the VPC peering connection, creating a VPC endpoint service in Account B and an interface VPC endpoint in Account A. After the migration, the web tier cannot connect to the application tier. The security groups and NACLs allow the traffic. Which of the following is the MOST likely cause of the connectivity issue?
⚠ Common exam trap
Many exam-takers assume PrivateLink transparently forwards traffic to the original NLB IP, but in reality the consumer must use the endpoint's DNS name or IP, not the service's original IP.
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 web tier instances are using the private IP address of the Network Load Balancer instead of the VPC endpoint DNS name or private IP addresses.
After migrating from VPC peering to AWS PrivateLink, the web tier instances must use the VPC endpoint's DNS name or its private IP addresses to connect to the application tier. The VPC endpoint provides a different set of IP addresses (from the subnet in Account A) than the NLB's private IP, which remains in Account B. Since the web tier continues to send traffic to the NLB's private IP, the packets are not routed through the VPC endpoint, causing connectivity failure because the VPC peering connection is no longer available.
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 web tier instances are using the private IP address of the Network Load Balancer instead of the VPC endpoint DNS name or private IP addresses.
Why this is correct
Correct; after migrating to PrivateLink, the web tier should use the VPC endpoint's DNS name or its assigned private IPs, not the NLB's IP directly.
- ✗
The route tables in Account A's VPC do not have a route to the VPC endpoint service in Account B.
Why it's wrong here
Incorrect; routes to VPC endpoints are managed automatically by the endpoint, not manually added.
- ✗
The VPC endpoint in Account A requires an Internet Gateway (IGW) to route traffic to the VPC endpoint service in Account B.
Why it's wrong here
Incorrect; PrivateLink uses private IPs and does not require IGW.
- ✗
The Network Load Balancer in Account B is not configured to support TCP traffic.
Why it's wrong here
Incorrect; NLB supports TCP by default.
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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.