DBS-C01 Monitoring and Troubleshooting Practice Question

Correct answer & explanation

Security groups are stateful, meaning that if an outbound rule allows traffic, the return traffic is automatically allowed. However, inbound traffic must be explicitly permitted by the target security group. In this scenario, the EC2 instance's security group allows outbound traffic to the Redis cluster's security group on port 6379, but the Redis cluster's security group does not have an inbound rule allowing traffic from the EC2 security group on that port. Therefore, the connection is blocked, causing timeout errors. Option A is incorrect because network ACLs are stateless and would affect both inbound and outbound traffic, but the issue is specifically with security groups. Option C is incorrect because subnet route tables control network routing, not port-level access. Option D is incorrect because ElastiCache for Redis clusters are accessible within the same VPC if proper security group rules are configured.