Attaching ENI on eth0 on EC2 instance startup

Question

I need to attach a static private IP to an instance in an ASG. Since I can't specify it in the Launch Template, I'm using a user-data script to attach an existing ENI on instance startup. It works fine with --device-index 1 but then I get a total of two IPs.

Why is it not possible to have --device-index 0 to attach only the IP address I need?

Answer 1

According to the AWS Certified Solutions Architect Study Guide,

1. "Every instance must have a primary.network interface"
2. "You can't remove the primary ENI from an instance, and you can't change its su.net."

My best guess is that by the time you run your user-data script, you already have a primary.network interface attached.