You have an EC2 Instance in a VPC which has 2 AZ’s. The EC2 Instance has an ENI with a public and private IP address. The EC2 Instance is hosting a web server. This web server connects to a database RDS Instance. There is a backup web server available on another subnet. You have initiated a disaster recovery scenario and detached the ENI from it`s original instance and attached it to the backup web server`s instance in the other subnet. When you try to use the web application, it now seems that the web server cannot connect to the database server. Which of the following could be the underlying issue?

Answer options:

A.The database server needs to get a new public IP to work with the ENI.
B.The security group for the database is blocking the traffic from the webserver subnet
C.The instance needs to be restarted so that it can start using the ENI
D.The backup web server needs to get a new public IP to work with the ENI.

Answer – B
If you already had a Security Group that was allowing traffic from the original private IP of the ENI, this would not be in effect when the ENI is moved to the newer subnet.
Hence the Security Group for the database would ideally need to be modified to ensure the traffic can flow from the new web server to the database server.
For more information on Elastic Network interfaces, please refer to below URL:
http://docs.aws.amazon.com/AWSEC2/latest/UserGuide/using-eni.html