If you have an AWS VPC with public and private subnets, it’s important to remember to select the public subnets when creating an Elastic Load Balancer. When the public subnets are chosen, replies from instances behind the load balancer are returned through the load balancer. If you instead select the private subnets then reply traffic is routed via the routing table for the private subnet. This results in asymmetric routing which can create a range of problems.