What happens when you attach an ENI of a different subnet to this EC2 instance?

An EC2 instance is connected to an ENI (Elastic Network Interface) in one subnet.

What happens when you attach an ENI of a different subnet to this EC2 instance?
A . The EC2 instance follows the rules of the older subnet
B . The EC2 instance follows the rules of both the subnets
C . Not possible, cannot be connected to 2 ENIs
D . The EC2 instance follows the rules of the newer subnet

Answer: B

Explanation:

AWS allows you create an elastic network interface (ENI), attach an ENI to an EC2 instance, detach an ENI from an EC2 instance and attach this ENI to another EC2 instance. The attributes of a network traffic follow the ENI which is attached to an EC2 instance or detached from an EC2 instance. When you move an ENI from one EC2 instance to another, network traffic is redirected to the new EC2 instance. You can create and attach additional ENIs to an EC2 instance.

Attaching multiple network interfaces (ENIs) to an EC2 instance is useful to:

Create a management network.

Use network and security appliances in your VPC.

Create dual-homed instances with workloads/roles on distinct subnets

Create a low-budget, high-availability solution.

Reference: http://docs.aws.amazon.com/AWSEC2/latest/UserGuide/using-eni.html