A global airline company has set up multiple VPCs in multiple regions for its three-tier application which is used for ticketing purposes by agents. Recently the IT team of this company has developed a new ticketing application which they need to evaluate on the test setup. For this, a new TEST VPC needs to be created & EC2 instances with the new applications need to be launched in this VPC. A Junior Engineer who was responsible for implementing this new setup did not specify Security Groups on all-new EC2 instances launched within TEST VPC. How will this impact communication between instances in TEST VPC?

Answer options:

A.Will allow all inbound traffic from instance within the same Security Groups & will allow all outbound traffic from the instance.
B.Will deny all inbound & outbound traffic from each instance.
C.Will allow all outbound traffic from instance within the same Security Groups & will allow all inbound traffic from the instance.
D.Will permit all inbound & outbound traffic between instances.

Correct Answer – A
When no Security Groups are specified during launch, the instance will be launched in default Security Groups for that VPC. In the case of default Security Groups, it will allow all inbound traffic from all instances with that Security Groups & allow all outbound traffic from the instance.
Option B is incorrect as with no Security Groups specified, a default Security Groups will be applied which do not deny all inbound & outbound traffic but all inbound traffic from instance within the same Security Groups & allows all outbound traffic.
Option C is incorrect as Default Security Groups allow all outbound traffic irrespective of the same Security Groups & do not allow all inbound traffic.
Option D is incorrect as for inbound traffic default Security Groups will allow traffic from an instance with the same Security Groups & will deny all other traffic.
For more information on using Default Security Groups, refer to the following URLs,
https://docs.aws.amazon.com/vpc/latest/userguide/VPC_SecurityGroups.html