AWS Elastic Load Balancing (ELB) automatically distributes incoming traffic to your application to multiple EC2 instances that are attached to your Elastic Load Balancer. At any time, Elastic Load Balancing detects the unhealthy instances in the pool, and distributes the incoming traffic only to the healthy instances until the unhealthy ones are restored.

In order to achieve greater fault tolerance and thus higher availability, it is recommended to distribute your instances in different geographical zones so that if all the instances in a single datacenter are not healthy, as may occur when there is an outage, your application will run in a data center in a different zone.

Identification

 Our identification of the issue is based on the following data - We have checked the configuration of your ELB and discovered that your servers are not balanced across multiple availability zones.

Instructions

 You should attach to the Elastic Load Balancer, instances that are located in at least two different availability zones.

How to Attach Instances to an Elastic Load Balancer

The following procedure describes how to attach instances to your ELB.

It is recommended that you add instances from a variety of availability zones to your ELB. Amazon recommends maintaining the same number of instances in each availability zone, as the ELB distributes traffic equally across availability zones.

5 steps to attach instances to the elastic load balancers - 

1. Launch the AWS Management Console.
2. In the left navigation pane, click Load Balancers. The Load Balancers pane is displayed.
3. In the Load Balancers pane, select the checkbox of the load balancer to which you want to add instances. The bottom pane displays the selected load balancer details.
4. Click the Instances tab. In the upper right corner of the Instances box, click the small plus/minus icon. You may have to scroll to the right to see the icon.
5. Select the checkboxes of the instances that you want to add to your load balancer, and then click Save.

Keywords: amazon aws availability zones, AWS Elastic Load Balancing, elb,  EC2 instances, aws management console.

Additional Relevant Resources

Get Started with Elastic Load Balancing

Configuring Elastic Load Balancing

AWS Auto-Scaling and ELB with Reliable Root Domain Handling

Presentation: Build powerful web applications in the AWS Cloud : A Love Story by Jinesh Varia