Register instances with your Classic Load Balancer

Registering an EC2 instance adds it to your load balancer. The load balancer continuously monitors the health of registered instances in its enabled Availability Zones, and routes requests to the instances that are healthy. If demand on your instances increases, you can register additional instances with the load balancer to handle the demand.

Deregistering an EC2 instance removes it from your load balancer. The load balancer stops routing requests to an instance as soon as it is deregistered. If demand decreases, or you need to service your instances, you can deregister instances from the load balancer. An instance that is deregistered remains running, but no longer receives traffic from the load balancer, and you can register it with the load balancer again when you are ready.

When you deregister an instance, Elastic Load Balancing waits until in-flight requests have completed if connection draining is enabled. For more information, see Configure connection draining for your Classic Load Balancer.

If your load balancer is attached to an Auto Scaling group, instances in the group are automatically registered with the load balancer. If you detach a load balancer from your Auto Scaling group, the instances in the group are deregistered.

Elastic Load Balancing registers your EC2 instance with your load balancer using its IP address.

[EC2-VPC] When you register an instance with an elastic network interface (ENI) attached, the load balancer routes requests to the primary IP address of the primary interface (eth0) of the instance.

Register an instance

When you are ready, register your instance with your load balancer. If the instance is an in Availability Zone that is enabled for the load balancer, the instance is ready to receive traffic from the load balancer as soon as it passes the required number of health checks.

To register your instances using the AWS CLI

Use the following register-instances-with-load-balancer command:

aws elb register-instances-with-load-balancer --load-balancer-name my-loadbalancer --instances i-4e05f721

The following is an example response that lists the instances registered with the load balancer:

{
    "Instances": [
        {
            "InstanceId": "i-315b7e51"
        }, 
        {
            "InstanceId": "i-4e05f721"
        }
    ]
}

View the instances registered with a load balancer

Use the following describe-load-balancers command to list the instances registered with the specified load balancer:

aws elb describe-load-balancers --load-balancer-names my-load-balancer --output text --query "LoadBalancerDescriptions[*].Instances[*].InstanceId"

The following is example output:

i-e905622e
i-315b7e51
i-4e05f721

Determine the load balancer for a registered instance

Use the following describe-load-balancers command to get the name of the load balancer to which the specified instance is registered:

aws elb describe-load-balancers --output text --query "LoadBalancerDescriptions[?Instances[?InstanceId=='i-e905622e']].[LoadBalancerName]"

The following is example output:

my-load-balancer

Deregister an instance

You can deregister an instance from your load balancer if you no longer need the capacity or if you need to service the instance.

If your load balancer is attached to an Auto Scaling group, detaching the instance from the group also deregisters it from the load balancer. For more information, see Detach EC2 instances from your Auto Scaling group in the Amazon EC2 Auto Scaling User Guide.

To deregister your instances using the AWS CLI

Use the following deregister-instances-from-load-balancer command:

aws elb deregister-instances-from-load-balancer --load-balancer-name my-loadbalancer --instances i-4e05f721

The following is an example response that lists the remaining instances registered with the load balancer:

{
    "Instances": [
        {
            "InstanceId": "i-315b7e51"
        }
    ]
}