Ensure that the EC2 instances registered to your Amazon Elastic Load Balancing (ELB) are evenly distributed across all Availability Zones (AZs) in order to improve the ELBs configuration reliability. To route incoming requests evenly across the registered EC2 instances within the Availability Zones, the cross-zone load balancing feature must be enabled.
Having a balanced distribution of EC2 instances across all AZs will improve the availability and reliability of your load balanced applications. The more Availability Zones assigned and better the spread, the more redundancy and availability within your load balancing environment.
To determine if your ELB backend instances are distributed evenly across all assigned AZs, perform the following:
To equally distribute your existing ELB backend instances across all Availability Zones within the selected AWS region, you need to add new Availability Zones to the ELB configuration and migrate the registered instances between these AZs. To implement this strategy, perform the following actions:Note: As example, this section will explain how to add a new AZ named us-east-1c to the existing ELB configuration and migrate a Linux EC2 instance from us-east-1a to the newly added AZ, i.e. us-east-1c, within the US East (N. Virginia) region: