You can now select more than one subnet per availability zone in an Autoscaling group.
You no longer need to remove an existing subnet from an Autoscaling group in order to reconfigure the subnets. You can simply add more subnets without affecting your applications availability.
More details in the docs at:
AutoScaling Developer Guide
Multiple Subnets per Availability Zone with Auto Scaling groups
Paul Coady CEO BackSpace Technology LLC
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