How to Design for Availability Zone Failure with ALB, RDS Multi-AZ, and Auto Scaling
A company is deploying a web application across multiple Availability Zones in a single AWS Region. The application consists of an Application Load Balancer (ALB) in front of EC2 instances in an Auto Scaling group, and an Amazon RDS Multi-AZ database. The company needs to ensure that the application can survive the loss of an entire Availability Zone. Which THREE actions should they take? (Select THREE.)
Quick Answer
The answer is to configure the Auto Scaling group to launch instances in at least two Availability Zones, enable Multi-AZ deployment for the RDS database, and ensure the Application Load Balancer is also configured as cross-zone load balancing across those same AZs. This trio of actions is correct because surviving availability zone failure requires redundancy at every layer: the Auto Scaling group distributes EC2 capacity across AZs so that if one fails, the remaining zones absorb traffic; RDS Multi-AZ provides a synchronous standby in a different AZ for automatic failover; and the ALB, when enabled for cross-zone, can route requests to healthy targets in surviving AZs without manual intervention. On the AWS Certified Advanced Networking Specialty ANS-C01 exam, this scenario tests your understanding of layered high-availability design—a common trap is assuming the ALB alone handles AZ failure, but it only distributes traffic; it cannot create instances or fail over the database. A useful memory tip is “three layers of survival: compute, database, and load balancing—each must span at least two AZs.”
⚠ Common exam trap
AWS often tests the misconception that simply enabling cross-zone load balancing on the ALB (Option C) alone is sufficient for AZ resilience, but without the Auto Scaling group spanning multiple AZs (Option B), there would be no healthy targets in the surviving AZs to route traffic to.
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
✓
Configure the Auto Scaling group to launch instances in at least two Availability Zones
Configuring the Auto Scaling group to launch instances in at least two Availability Zones ensures that if one AZ fails, the remaining AZ(s) still have healthy EC2 instances to serve traffic. This is a fundamental requirement for multi-AZ high availability, as the Auto Scaling group will automatically replace failed instances in the remaining 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 larger EC2 instance types to handle the load
Why it's wrong here
Larger instances do not provide AZ resilience.
- ✓
Configure the Auto Scaling group to launch instances in at least two Availability Zones
Why this is correct
Ensures instances are spread across AZs.
- ✓
Configure the Application Load Balancer to be internet-facing and enable cross-zone load balancing
Why this is correct
Cross-zone load balancing helps distribute traffic across AZs.
- ✓
Enable Multi-AZ deployment for the RDS database
Why this is correct
Multi-AZ provides automatic failover to a standby in another AZ.
- ✗
Deploy a single NAT Gateway in one AZ for outbound traffic
Why it's wrong here
Single NAT Gateway is a single point of failure.
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Same concept, more angles
2 more ways this is tested on ANS-C01
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 deploying a multi-tier web application across three Availability Zones in a single AWS Region. The web tier must be fault-tolerant and scale horizontally. Which network design provides the highest availability and scalability?
medium- A.Deploy a single EC2 instance in one Availability Zone with an Elastic IP address.
- ✓ B.Use an Application Load Balancer in front of an Auto Scaling group spanning three Availability Zones.
- C.Deploy EC2 instances in a single Availability Zone and use Route 53 weighted routing.
- D.Use a Network Load Balancer with a single EC2 instance in each AZ.
Why B: An Application Load Balancer (ALB) distributes incoming HTTP/HTTPS traffic across multiple targets, and when combined with an Auto Scaling group that spans three Availability Zones, it provides both horizontal scalability and fault tolerance. If one AZ fails, the ALB automatically routes traffic only to healthy instances in the remaining AZs, ensuring high availability. This design meets the requirement for a multi-tier web application that must scale horizontally and tolerate failures.
Variation 2. A company has a VPC with multiple subnets across two Availability Zones. They are designing a highly available web application using an Application Load Balancer (ALB) and EC2 instances in an Auto Scaling group. Which of the following is the most resilient and cost-effective design for the network layer?
medium- A.Deploy a Network Load Balancer (NLB) in each AZ and associate them with a single ALB.
- B.Deploy an ALB in one AZ and EC2 instances in the same AZ.
- ✓ C.Deploy a single ALB with subnets in two AZs and enable cross-zone load balancing.
- D.Deploy an ALB in each AZ and use Route 53 weighted routing.
Why C: Deploying a single Application Load Balancer (ALB) with subnets in two Availability Zones (AZs) and enabling cross-zone load balancing provides high availability and fault tolerance at the network layer. The ALB automatically distributes incoming traffic across healthy targets in all enabled AZs, eliminating the need for multiple load balancers and reducing costs while maintaining resilience. This design leverages the ALB's native ability to handle AZ failures by routing traffic only to healthy AZs, making it both resilient and cost-effective.
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.