AWS Certified Solutions Architect – Professional — Question 427

A solutions architect has an operational workload deployed on Amazon EC2 instances in an Auto Scaling group. The VPC architecture spans two Availability
Zones (AZ) with a subnet in each that the Auto Scaling group is targeting. The VPC is connected to an on-premises environment and connectivity cannot be interrupted. The maximum size of the Auto Scaling group is 20 instances in service. The VPC IPv4 addressing is as follows:

VPC CIDR: 10.0.0.0/23 -

AZ1 subnet CIDR: 10.0.0.0/24 -

AZ2 subnet CIDR: 10.0.1.0/24 -
Since deployment, a third AZ has become available in the Region. The solutions architect wants to adopt the new AZ without adding additional IPv4 address space and without service downtime.
Which solution will meet these requirements?

Answer options

• A. Update the Auto Scaling group to use the AZ2 subnet only. Delete and re-create the AZ1 subnet using half the previous address space. Adjust the Auto Scaling group to also use the new AZ1 subnet. When the instances are healthy, adjust the Auto Scaling group to use the AZ1 subnet only. Remove the current AZ2 subnet. Create a new AZ2 subnet using the second half of the address space from the original AZ1 subnet. Create a new AZ3 subnet using half the original AZ2 subnet address space, then update the Auto Scaling group to target all three new subnets.
• B. Terminate the EC2 instances in the AZ1 subnet. Delete and re-create the AZ1 subnet using half the address space. Update the Auto Scaling group to use this new subnet. Repeat this for the second AZ. Define a new subnet in AZ3, then update the Auto Scaling group to target all three new subnets.
• C. Create a new VPC with the same IPv4 address space and define three subnets, with one for each AZ. Update the existing Auto Scaling group to target the new subnets in the new VPC.
• D. Update the Auto Scaling group to use the AZ2 subnet only. Update the AZ1 subnet to have the previous address space. Adjust the Auto Scaling group to also use the AZ1 subnet again. When the instances are healthy, adjust the Auto Scaling group to use the AZ1 subnet only. Update the current AZ2 subnet and assign the second half of the address space from the original AZ1 subnet. Create a new AZ3 subnet using half the original AZ2 subnet address space, then update the Auto Scaling group to target all three new subnets.

Correct answer: A

Explanation

Option A is the correct solution because it allows the subnets to be resized sequentially without causing service downtime, as the Auto Scaling group is shifted to the active subnet while the other is deleted and recreated with a smaller CIDR block. Option B is incorrect because terminating instances directly causes service disruption. Option C is incorrect because creating a new VPC would disrupt the existing connection to the on-premises environment, and Option D is incorrect because you cannot modify the CIDR block of an existing subnet without deleting it first.