AWS Certified Advanced Networking Specialty Practice Exam

Which of the following is a common cause for issues with visibility between EC2 instances in different subnets?

1. Inadequate EC2 instance types.

2. Overlapping security group rules.

3. Incorrect routing tables.

4. Using different region VPCs.

The correct answer is: Incorrect routing tables.

A common cause for issues with visibility between EC2 instances in different subnets is related to incorrect routing tables. In Amazon Virtual Private Cloud (VPC), each subnet is associated with a routing table that determines how network traffic is directed. If the routing tables do not correctly route traffic between subnets, instances in those subnets will not be able to communicate with each other. For instance, if the routes are not properly defined to allow traffic to flow between the subnets, or if the destination for the traffic is not reachable due to missing or incorrect entries, this can lead to connectivity issues. It is essential to ensure that the routing tables include the right routes allowing multiple subnets to communicate, especially when using private IP addresses. In contrast, inadequate EC2 instance types would affect performance but not visibility or connectivity between instances. Overlapping security group rules could complicate access control, but if configured correctly, instances might still reach each other. Using different region VPCs is a scenario that would inherently prevent visibility altogether, as resources in different regions are segregated; it’s not simply a visibility issue within the same environment. Thus, focusing on the routing tables is critical for resolving such visibility issues effectively.