Application Load Balancer not freeing up old private ip after changing subnet

Question

Hi, I have an ALB deployed in 2 AZs. I want retire the subnets it has been using. I successfully changed the subnets to the newer ones but it has been 6 days since the subnet update I can still see an ENI owned by this ALB running in the old subnet with the status "in use". I can not detach or delete it.

I tried to remove that AZ for my ALB for several minutes and later add the AZ back with the new subnet.  However, that ENI with ip from old subnet is still there handing traffic.

Also I enabled VPC Flow logs for that subnet, I can see many requests going between this specific ENI to all the target groups it has associated with, both healthcheck and normal user requests.
How can i get rid of it? I have target groups associated with this ALB with attributes "stickiness" set to "ON", Is there the reason for this behavior?

Answer

Thanks for the detailed description.

First of all, when you update the subnets, the ALB nodes (ENIs) are replaced with new nodes in new subnets. Replaced node that has been removed from service will first be removed from DNS so that new traffic will not be sent to it. Then it will enter a grace period waiting for traffic to drain, and existing requests to be completed, after which it will be shutdown. That said, if your clients continue to send requests to the replaced nodes (e.g. through a long-lived connection), the node takes longer time to be drained.

Stickiness setting on the other hand is to make sure any ALB node forwards to the same target or target group, which does not contribute to the current observation.

Application Load Balancer not freeing up old private ip after changing subnet

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