Configure cross-zone load balancing for your Classic Load Balancer

With cross-zone load balancing, each load balancer node for your Classic Load Balancer distributes requests evenly across the registered instances in all enabled Availability Zones. If cross-zone load balancing is disabled, each load balancer node distributes requests evenly across the registered instances in its Availability Zone only. For more information, see Cross-zone load balancing in the Elastic Load Balancing User Guide.

Cross-zone load balancing reduces the need to maintain equivalent numbers of instances in each enabled Availability Zone, and improves your application's ability to handle the loss of one or more instances. However, we still recommend that you maintain approximately equivalent numbers of instances in each enabled Availability Zone for higher fault tolerance.

For environments where clients cache DNS lookups, incoming requests might favor one of the Availability Zones. Using cross-zone load balancing, this imbalance in the request load is spread across all available instances in the Region, reducing the impact of misbehaving clients.

When you create a Classic Load Balancer, the default for cross-zone load balancing depends on how you create the load balancer. With the API or CLI, cross-zone load balancing is disabled by default. With the AWS Management Console, the option to enable cross-zone load balancing is selected by default. After you create a Classic Load Balancer, you can enable or disable cross-zone load balancing at any time.

Enable cross-zone load balancing

You can enable cross-zone load balancing for your Classic Load Balancer at any time.

To enable cross-zone load balancing using the console

1. Open the Amazon EC2 console at https://console.aws.amazon.com/ec2/.

2. On the navigation pane, under Load Balancing, choose Load Balancers.

3. Choose the name of the load balancer to open its detail page.

4. On the Attributes tab, choose Edit.

5. On the Edit load balancer attributes page, in the Availability Zone routing configuration section, enable Cross-zone load balancing.

6. Choose Save changes.

To enable cross-zone load balancing using the AWS CLI

1. Use the following modify-load-balancer-attributes command to set the CrossZoneLoadBalancing attribute of your load balancer to true:

   aws elb modify-load-balancer-attributes --load-balancer-name my-loadbalancer --load-balancer-attributes "{\"CrossZoneLoadBalancing\":{\"Enabled\":true}}"

   The following is an example response:

   {
      "LoadBalancerAttributes": {
        "CrossZoneLoadBalancing": {
            "Enabled": true
          }
      },
      "LoadBalancerName": "my-loadbalancer"
    }
   

2. (Optional) Use the following describe-load-balancer-attributes command to verify that cross-zone load balancing is enabled for your load balancer:

   aws elb describe-load-balancer-attributes --load-balancer-name my-loadbalancer

   The following is an example response:

   {
       "LoadBalancerAttributes": {
           "ConnectionDraining": {
               "Enabled": false, 
               "Timeout": 300
           }, 
           "CrossZoneLoadBalancing": {
               "Enabled": true
           }, 
           "ConnectionSettings": {
               "IdleTimeout": 60
           }, 
           "AccessLog": {
               "Enabled": false
           }
       }
   }

Disable cross-zone load balancing

You can disable the cross-zone load balancing option for your load balancer at any time.

To disable cross-zone load balancing using the console

1. Open the Amazon EC2 console at https://console.aws.amazon.com/ec2/.

2. On the navigation pane, under Load Balancing, choose Load Balancers.

3. Choose the name of the load balancer to open its detail page.

4. On the Attributes tab, choose Edit.

5. On the Edit load balancer attributes page, in the Availability Zone routing configuration section, disable Cross-zone load balancing.

6. Choose Save changes.

To disable cross-zone load balancing, set the CrossZoneLoadBalancing attribute of your load balancer to false.

To disable cross-zone load balancing using the AWS CLI

1. Use the following modify-load-balancer-attributes command:

   aws elb modify-load-balancer-attributes --load-balancer-name my-loadbalancer --load-balancer-attributes "{\"CrossZoneLoadBalancing\":{\"Enabled\":false}}"

   The following is an example response:

   {
      "LoadBalancerAttributes": {
        "CrossZoneLoadBalancing": {
            "Enabled": false
          }
      },
      "LoadBalancerName": "my-loadbalancer"
    }

2. (Optional) Use the following describe-load-balancer-attributes command to verify that cross-zone load balancing is disabled for your load balancer:

   aws elb describe-load-balancer-attributes --load-balancer-name my-loadbalancer

   The following is an example response:

   {
       "LoadBalancerAttributes": {
           "ConnectionDraining": {
               "Enabled": false, 
               "Timeout": 300
           }, 
           "CrossZoneLoadBalancing": {
               "Enabled": false
           }, 
           "ConnectionSettings": {
               "IdleTimeout": 60
           }, 
           "AccessLog": {
               "Enabled": false
           }
       }
   }