Redshift cluster subnet groups should have subnets from multiple Availability Zones

How to fix it

Add subnets from at least two Availability Zones to the cluster subnet group. The call replaces the whole subnet list, so pass the existing subnet IDs along with the new one, and every subnet in one group must share the same network ACL rules and route table routes. If Redshift.18 is open on a cluster using this group, add three zones now, because Multi-AZ needs three.

In the console

Redshift → Provisioned clusters dashboard → Configurations → Subnet groups → the subnet group → Edit

With the AWS CLI

aws redshift modify-cluster-subnet-group --cluster-subnet-group-name <subnet-group-name> --subnet-ids <subnet-id-1> <subnet-id-2> <subnet-id-3> --region <region>

With Terraform

resource "aws_redshift_subnet_group" "this" {
  name       = var.subnet_group_name
  subnet_ids = var.subnet_ids
}