Classic Load Balancer should span multiple Availability Zones

How to fix it

Attach subnets so the Classic Load Balancer is enabled in at least two Availability Zones, which is the threshold this finding uses. Adding a zone keeps the load balancer's name and DNS name. The new zone only carries traffic once instances are registered in it, and cross-zone load balancing (ELB.9) should be on, or traffic is split evenly between zones whatever the instance counts.

In the console

EC2 -> Load balancers -> the load balancer -> Network mapping -> Edit subnets

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"
  }
}