Classic Load Balancer should span multiple Availability Zones

How to fix it

• Interrupts service: Register instances in the new zone before adding its subnet: zones get equal shares of requests, and without cross-zone load balancing (ELB.9) that share has nowhere to go.

In the console

EC2 → Load balancers → the load balancer → Network mapping → Edit subnets

Network mapping

One subnet in each of at least two Availability Zones, in zones where instances are registered

With the AWS CLI

aws elb attach-load-balancer-to-subnets --load-balancer-name <load-balancer-name> --subnets <subnet-id> --region <region>

With Terraform

resource "aws_elb" "this" {
  subnets = var.subnet_ids

  listener {
    lb_port           = 80
    lb_protocol       = "http"
    instance_port     = 80
    instance_protocol = "http"
  }
}