Subnet auto-discovery

By default, the AWS Load Balancer Controller (LBC) auto-discovers network subnets that it can create AWS Network Load Balancers (NLB) and AWS Application Load Balancers (ALB) in. ALBs require at least two subnets across Availability Zones by default, set Feature Gate ALBSingleSubnet to "true" allows using only 1 subnet for provisioning ALB. NLBs require one subnet. The subnets must be tagged appropriately for auto-discovery to work. The controller chooses one subnet from each Availability Zone. During auto-discovery, the controller considers subnets with at least eight available IP addresses. In the case of multiple qualified tagged subnets in an Availability Zone, the controller chooses the first one in lexicographical order by the subnet IDs. For more information about the subnets for the LBC, see Application Load Balancers and Network Load Balancers. If you used eksctl or an Amazon EKS AWS CloudFormation template to create your VPC after March 26, 2020, then the subnets are tagged appropriately when they're created. For more information about the Amazon EKS AWS CloudFormation VPC templates, see Creating a VPC for your Amazon EKS cluster.

Public subnets

Public subnets are used for internet-facing load balancers. These subnets must have the following tags:

Key	Value
kubernetes.io/role/elb	1 or ``

Private subnets

Private subnets are used for internal load balancers. These subnets must have the following tags:

Key	Value
kubernetes.io/role/internal-elb	1 or ``

Common tag

In version v2.1.1 and older of the LBC, both the public and private subnets must be tagged with the cluster name as follows:

Key	Value
kubernetes.io/cluster/${cluster-name}	owned or shared

${cluster-name} is the name of the Kubernetes cluster.

The cluster tag is not required in versions v2.1.2 to v2.4.1, unless a cluster tag for another cluster is present.

With versions v2.4.2 and later, you can disable the cluster tag check completely by specifying the feature gate SubnetsClusterTagCheck=false