Designing cross-region replication for AWS EC2 instances with an AWS Application Load Balancer (ALB) involves several steps to ensure high availability, fault tolerance, and efficient traffic distribution across regions. Here's a step-by-step approach to achieve this:

1. Setup of AWS Regions and Availability Zones
Select Regions: Choose the AWS regions in which you want to replicate your EC2 instances. Ensure these regions support the services you need.
Availability Zones: Within each region, select multiple Availability Zones (AZs) for higher fault tolerance.
2. EC2 Instance Replication
AMI Creation: Create an Amazon Machine Image (AMI) of your EC2 instance in the primary region.
AMI Copy: Copy the AMI to the secondary region(s).
Instance Launch: Launch EC2 instances in the secondary region(s) using the copied AMI.
3. Data Synchronization
Database Replication: If your application uses a database, set up cross-region database replication.
Storage Synchronization: Use Amazon S3 with Cross-Region Replication (CRR) for any required S3 bucket data synchronization.
File System Consistency: For shared file systems, consider AWS services like EFS or FSx, which can be replicated across regions.
4. Load Balancing and Traffic Distribution
Regional ALBs: Set up Application Load Balancers in each region to distribute traffic to the EC2 instances in their respective regions.
Route 53: Use Amazon Route 53 for DNS and traffic management:
Geolocation Routing: Route users to the nearest region for better performance.
Health Checks: Implement health checks to monitor the health of the instances across regions.
Failover Routing: Configure failover routing to redirect traffic from the primary to the secondary region in case of an outage.
5. Auto-Scaling
Auto Scaling Groups: Implement Auto Scaling Groups in each region to automatically adjust the number of EC2 instances based on demand.
6. Security and Compliance
Security Groups and NACLs: Replicate security group and network ACL settings across regions.
IAM Roles and Policies: Ensure consistent IAM roles and policies for EC2 instances in all regions.