How to Understand AWS DMS Subnet Group Selection Process

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When setting up a DMS (Database Migration Service) replication subnet group, it’s important to include at least two subnets located in different Availability Zones within the same VPC. This setup helps ensure your replication runs smoothly and stays available even if one zone experiences issues.

AWS DMS doesn’t specify a particular method for choosing subnets from your group. When you create the subnet group, you simply select the subnets, and AWS automatically determines the VPC associated with each subnet, since each is linked to only one VPC.

Unlike some other AWS services that have clear rules for choosing subnets—like Amazon EMR, which considers available IP addresses—DMS doesn’t give detailed guidance on how it picks between subnets inside a group.

If you’re worried about running out of IP addresses, especially since subnets are shared with other services like EKS and might have limited IPs, take these steps:

– Regularly check how many IP addresses are being used in your subnets.
– Make sure your subnets have enough free IPs for your needs.
– Opt for larger CIDR blocks when creating subnets to expand available IP space.
– Always include at least two subnets in different zones to improve the chances of having open IP addresses.

For better control over where your DMS resources are placed, you can create separate subnet groups with specific subnets dedicated to DMS. This way, you’ll have more predictable IP allocation and can better manage your migration setup.

Sources:
– Setting up a network for a replication instance – AWS DMS official guide
– Enhancing workload resilience with Amazon EMR features
– Creating subnet groups for AWS DMS migrations