AWS DMS Replication Instances on different Availability Zones Connection Issue

Question

I am facing an issue with AWS DMS Replication Instance - it is not working on a specific AZ.

Below explains the scenario and setup:

AWS Account #1

• MySQL RDS (Multi-AZ)

AWS Account #2

• DMS Replication Instance (on Availability Zone - A)
• DMS Replication Instance (on Availability Zone - B)
• EC2 Instance (on Availability Zone A)

Both AWS accounts are connected fine with a Transit Gateway between them.

The RDS has a Security Group attached to allow communication from Account #2 to Account #1.

When connecting to the RDS through the EC2 instance, it works fine. When testing the DMS endpoint using the Replication Instance (AZ-A) it does not work, but when testing the DMS endpoint using the Replication Instance (AZ-B) it works fine.

I checked every single thing, from Security Groups, to Network ACLs, to Transit Gateways - all communication (inbound rules) are fine but still not sure what's the issue.

Any help or tips on the solution would be great.

Thanks,

Answer 1

Found the fix!

There was a missing route table entry pointing to the Transit Gateway!

The following entry worked for me:

Destination = 0.0.0.0/0

Target = {PUT YOUR TRANSIT GATEWAY ID HERE, eg tgw-xxxxx}