A company runs a highly available, scalable application on EC2 instances that are managed by an Auto Scaling group. The network team is rolling out IPv6 support in stages. In the first stage, they enabled IPv6 on public dual-stack Network Load Balancers (NLBs) whose target groups point to the Auto Scaling groups of the application EC2 instances. During testing, IPv6 requests reach the NLBs but do not reach the backend servers. What is the root cause?

The correct answer is that the route tables for the EC2 subnets do not have IPv6 routing configured. Even though the NLB is dual-stack and receives IPv6 traffic, the backend EC2 instances need to be in subnets that are IPv6-enabled and have route tables configured to direct IPv6 traffic. Without IPv6 routes in the EC2 subnets' route tables, the IPv6 traffic cannot reach the instances. Incorrect options: The subnets where the EC2 instances are deployed do not have IPv6 addresses configured: While necessary, this is not the root cause described. The problem states requests reach the NLB, implying the NLB's configuration is fine, and the issue is reaching the backend servers. If the EC2 instances didn't have IPv6 addresses, the NLB couldn't forward IPv6 traffic to them. The route tables for the NLB subnets do not have IPv6 routing configured: The problem states IPv6 requests reach the NLBs, which means the NLB's subnet routing is functional for IPv6. The security groups that are associated with the NLBs do not allow IPv6 traffic: NLBs do not use security groups. Security groups are associated with the target EC2 instances.