Update the Availability Zones for your Application Load Balancer
You can enable or disable the Availability Zones for your load balancer at any time. After you enable an Availability Zone, the load balancer starts routing requests to the registered targets in that Availability Zone. Application Load Balancers have cross-zone load balancing on by default, resulting in requests being routed to all registered targets across all Availability Zones. When cross-zone load balancing is off, the load balancer only routes request to targets in the same Availability Zone. For more information, see Cross-zone load balancing. Your load balancer is most effective if you ensure that each enabled Availability Zone has at least one registered target.
After you disable an Availability Zone, the targets in that Availability Zone remain registered with the load balancer, but the load balancer will not route requests to them.
To update Availability Zones using the console
Open the Amazon EC2 console at https://console.aws.amazon.com/ec2/
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On the navigation pane, choose Load Balancers.
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Select the load balancer.
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On the Network mapping tab, choose Edit subnets.
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To enable an Availability Zone, select its check box and select one subnet. If there is only one available subnet, it is selected for you.
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To change the subnet for an enabled Availability Zone, choose one of the other subnets from the list.
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To disable an Availability Zone, clear its check box.
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Choose Save changes.
To update Availability Zones using the AWS CLI
Use the set-subnets command.
