You are managing a legacy application inside VPC with hard-coded IP addresses in its configuration. Which mechanisms will allow the application to failover to new instances without much reconfiguration?

Answer options:

A.Use the traffic manager to route the traffic to the failover instances.
B.Create a secondary ENI that can be moved to the failover instance.
C.Use Route53 health checks to reroute the traffic to the failover instance.
D.Assign a secondary private IP address to the primary ENI of the failover instance.

Answer - B and D
Option A is incorrect because rerouting to a failover instance cannot be done through a Traffic Manager.
Option B is CORRECT because the attributes of a network interface follow it as it`s attached or detached from an instance and reattached to another instance. When you move a network interface from one instance to another, network traffic is redirected to the new instance.
Option C is incorrect because Route 53 cannot reroute the traffic to the failover instance with the same IP address.
Option D is CORRECT because you can have a secondary IP address that can be configured on the primary ENI of the failover instance.
Best Practices for Configuring Network Interfaces
You can attach a network interface to an instance when it`s running (hot attach), when it`s stopped (warm attach), or when the instance is being launched (cold attach).
You can detach secondary (ethN) network interfaces when the instance is running or stopped. However, you can`t detach the primary (eth0) interface.
You can attach a network interface in one subnet to an instance in another subnet in the same VPC; however, both the network interface and the instance must reside in the same Availability Zone.
When launching an instance from the CLI or API, you can specify the network interfaces to attach to the instance for both the primary (eth0) and additional network interfaces.
Launching an Amazon Linux or Windows Server instance with multiple network interfaces automatically configures interfaces, private IPv4 addresses, and route tables on the operating system of the instance.
A warm or hot attach of an additional network interface may require you to manually bring up the second interface, configure the private IPv4 address, and modify the route table accordingly. Instances running Amazon Linux or Windows Server automatically recognize the warm or hot attach and configure themselves.
Attaching another network interface to an instance (for example, a NIC teaming configuration) cannot be used as a method to increase or double the network bandwidth to or from the dual-homed instance.
If you attach two or more network interfaces from the same subnet to an instance, you may encounter networking issues such as asymmetric routing. If possible, use a secondary private IPv4 address on the primary network interface instead. For more information, see Assigning a Secondary Private IPv4 Address.
For more information on Network Interfaces, please visit the below URL-
http://docs.aws.amazon.com/AWSEC2/latest/UserGuide/using-eni.html