Description

Ensure that EC2 Auto Scaling groups are configured to launch instances across multiple Availability Zones within a Region, rather than being confined to a single Availability Zone.

Rationale

An Auto Scaling group confined to a single Availability Zone places all of its capacity at risk of a single zonal outage, power event, or network disruption, which can cause a complete service outage. Spanning multiple Availability Zones allows the group to continue serving traffic, at reduced capacity, if a single zone becomes unavailable, significantly improving the resilience of the workload.

Impact

  • Reduces the risk of a complete outage due to a single Availability Zone failure

  • Improves overall application resilience and availability

  • Requires subnets to be available in multiple zones and application architecture to tolerate cross-AZ operation

Default Value

There is no fixed default; the set of Availability Zones (via subnets) used by an Auto Scaling group is determined by the subnets specified at group creation, and a group can be created spanning only a single zone if only one subnet is selected.

Pre-Requisites

  • Subnets available in at least two Availability Zones within the target VPC

  • IAM permissions to update Auto Scaling group settings (autoscaling:UpdateAutoScalingGroup)

Test Plan

  1. Sign in to the AWS Management Console at https://console.aws.amazon.com.

  2. Navigate to EC2 > Auto Scaling Groups and select the group to review.

  3. Open the Details tab and review the Availability Zones / subnets configured for the group.

  4. Verify that at least two Availability Zones (via distinct subnets) are configured.

  5. If the group spans only one Availability Zone, follow the implementation steps.

Implementation Steps

  1. Sign in to the AWS Management Console at https://console.aws.amazon.com.

  2. Navigate to EC2 > Auto Scaling Groups and select the group to update.

  3. On the Details tab, locate the Network section and select Edit.

  4. Add subnets from one or more additional Availability Zones to the group's subnet selection.

  5. Choose Update to save the change.

  6. Verify subsequent scale-out activity launches instances across the newly added zones as expected.

Backout Plan

  1. Sign in to the AWS Management Console at https://console.aws.amazon.com.

  2. Navigate to EC2 > Auto Scaling Groups and select the affected group.

  3. On the Details tab, locate the Network section and select Edit.

  4. Remove the additional Availability Zone subnets, leaving only the original single zone.

  5. Choose Update to save the change.

Reference