Investigating High InterZone-In cost

Hello.

I don't know the exact configuration of the AWS resources within the VPC, so I don't know the exact details, but I think you can get some idea of ​​the communication between EC2 by enabling the VPC flow log.
This may allow you to investigate whether communication between AZs is occurring.
https://docs.aws.amazon.com/vpc/latest/userguide/flow-logs.html

"InterZone-In" costs, usually indicate an increase in data transfer between different Availability Zones (AZs). Since you mentioned infrastructure changes, it's likely that these changes are affecting the data transfer patterns within your AWS environment. Here are some steps to help you identify and manage the increased costs:

Review CloudWatch Metrics: NetworkIn/NetworkOut Metrics:ThatNetworkIn and NetworkOutof Data Transfer by AZ: Look for metrics that might indicate data transfer between AZs. CloudWatch does not provide a direct metric for inter-AZ data transfer, but you can infer this by looking at the traffic patterns.

Cost Breakdown: Use AWS Cost Explorer to get a detailed breakdown of your costs. Filter by service and look at the EC2-Other costs to see which resources are incurring the most charges. Data Transfer: Within Cost Explorer, you can filter and group costs by Usage Type to identify data transfer charges. Look for InterZone-In costs specifically.

Enable VPC Flow Logs: Use Athena or a third-party tool to analyze VPC Flow Logs. Look for patterns indicating large data transfers between different subnets/AZs. https://docs.aws.amazon.com/vpc/latest/userguide/flow-logs.html

Check Your Network Configuration: If you use placement groups, ensure that they are configured correctly. A misconfigured placement group could lead to unintended cross-AZ traffic.