AWS Subnet Migration Documentation

[mikegrima]

Is your feature request related to a problem?
Hello! I am looking for some documentation to add clarity around how the controller handles AWS subnet migrations.

For example, let's say:

1. I have a subnets named foo-az-1 and foo-az-2.
2. Both subnets are tagged kubernetes.io/role/elb = 1
3. I want to migrate the ALB to new subnets named: bar-az-1, and bar-az-2.

Based on the documentation here: https://kubernetes-sigs.github.io/aws-load-balancer-controller/latest/deploy/subnet_discovery/, it would seem that I just tag bar-az-1 and bar-az-2 with kubernetes.io/role/elb set to 1. However, it's not clear if that would work or what the migration would look like. For example, the documentation says:

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.

Is the solution to, at first, ensure that foo-az-1, foo-az-2, bar-az-1, and bar-az-2 are all tagged with kubernetes.io/role/elb set to 1? Then once that is completed, untag foo-az-1 and foo-az-2?

It's not clear what the controller would do in this case. I'm also concerned about downtime, if there would be any.

Describe the solution you'd like
Some clarity on the how subnet migrations take place.

Describe alternatives you've considered
See above.

[aravindsagar]

Hi, thanks for bringing this up. The proposed solution should work, as the controller would use the modify subnets API under the hood to change the subnets. Alternately, you can also use the subnet annotation instead of subnet discovery to explicitly specify the subnets to be used by the load balancer.