How to Reduce Cross-AZ Data Transfer Costs in a Multi-Tier Web Application
A company is deploying a multi-tier web application on AWS. The application must be highly available across three Availability Zones. The web tier runs on EC2 instances behind an Application Load Balancer (ALB). The application tier runs on EC2 instances behind a Network Load Balancer (NLB). The database tier uses a Multi-AZ RDS instance. To reduce cross-AZ data transfer costs, which design should be implemented?
Quick Answer
The answer is to place web and application tier instances in the same subnets across all three Availability Zones. This design is correct because it leverages the Application Load Balancer’s cross-zone load balancing (enabled by default) and the Network Load Balancer’s ability to route traffic to targets in the same AZ, ensuring that traffic between the ALB, NLB, and application instances stays within the same AZ whenever possible, directly reducing cross-AZ data transfer costs in a multi-tier web application. On the AWS Certified Solutions Architect Professional SAP-C02 exam, this question tests your understanding of how AWS bills for inter-AZ traffic and the subtle difference between ALB and NLB routing behavior—a common trap is assuming you must place all tiers in a single AZ or use separate subnets, which would either sacrifice high availability or increase costs. A useful memory tip: “Same subnet, same AZ, same bill—keep traffic local to save your dollar.”
⚠ Common exam trap
Test-takers frequently think placing all resources in a single AZ (Option C) is acceptable for cost savings, but the question explicitly requires high availability across three AZs, making that option invalid despite its cost advantage.
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
✓
Place web and application tier instances in the same subnets across all three AZs
Placing web and application tier EC2 instances in the same subnets across all three Availability Zones ensures that traffic between the ALB and NLB, as well as between the NLB and application instances, stays within the same AZ whenever possible. This design leverages the ALB's cross-zone load balancing behavior (enabled by default) and the NLB's ability to route traffic to targets in the same AZ, minimizing cross-AZ data transfer costs. AWS charges for data transfer between AZs, so keeping traffic within the same AZ reduces those costs while maintaining high availability across three AZs.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
Use AWS Global Accelerator to reduce data transfer costs
Why it's wrong here
Global Accelerator improves performance but does not reduce internal cross-AZ costs.
- ✗
Place all web tier instances in one AZ and all application tier instances in another AZ
Why it's wrong here
This increases cross-AZ traffic and costs.
- ✗
Use a single AZ for all tiers to avoid cross-AZ traffic
Why it's wrong here
Single AZ reduces high availability.
- ✓
Place web and application tier instances in the same subnets across all three AZs
Why this is correct
ALB and NLB can route to targets in the same AZ, reducing cross-AZ traffic.
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Same concept, more angles
2 more ways this is tested on SAP-C02
These questions test the same concept from different angles. Work through them to make sure you can recognise it however the exam phrases it.
Variation 1. A company is designing a multi-tier web application on AWS. The application requires high availability across multiple Availability Zones. Which AWS service should be used to distribute incoming traffic across multiple EC2 instances in different Availability Zones?
easy- A.AWS Global Accelerator
- ✓ B.Application Load Balancer
- C.AWS Direct Connect
- D.Amazon Route 53
Why B: The Application Load Balancer (ALB) operates at Layer 7 of the OSI model and is designed to distribute incoming HTTP/HTTPS traffic across multiple targets, such as EC2 instances, in different Availability Zones. By registering instances in multiple AZs and enabling cross-zone load balancing, the ALB provides high availability and fault tolerance for the multi-tier web application.
Variation 2. A company is designing a multi-tier web application on AWS. The application must be highly available and scale automatically based on traffic. The web tier runs on Amazon EC2 instances behind an Application Load Balancer. The application tier also uses EC2 instances. Which solution meets these requirements with the LEAST operational overhead?
medium- A.Use Amazon ECS with Fargate for both tiers, with an Application Load Balancer.
- B.Use AWS Global Accelerator with a single Auto Scaling group for both tiers.
- C.Use Network Load Balancer with Auto Scaling groups for both tiers.
- ✓ D.Use Auto Scaling groups for both web and application tiers, and route traffic through an Application Load Balancer.
Why D: Using Auto Scaling groups for both web and application tiers with an Application Load Balancer provides automatic scaling and high availability with minimal operational overhead. Auto Scaling groups handle instance health and scaling based on demand, while the ALB distributes HTTP traffic. Option A is incorrect because ECS with Fargate introduces container management complexity, increasing operational overhead compared to EC2 Auto Scaling groups. Option B is incorrect because AWS Global Accelerator does not provide automatic scaling for instances; it only improves traffic routing and latency. Option C is incorrect because a Network Load Balancer is designed for TCP/UDP traffic and does not natively support HTTP-layer features needed for a web application, and it does not integrate with Auto Scaling for automatic scaling as seamlessly as an ALB.
JA
Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This SAP-C02 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 SAP-C02 exam.