What is the best way to handle networking during a long-running cloud migration?

Hi everyone,

I'd like to get some opinions from people who have worked on large-scale cloud migrations.

Consider a migration where workloads are gradually moved from AWS to Azure over several weeks or months rather than being migrated in a single cutover.

During the migration period, both environments need to remain operational.

A simplified architecture could look like:

AWS workloads

|

| Existing connectivity

|

AWS Network

|

| Inter-cloud connectivity

|

Azure Network

|

|

Azure workloads

The challenge is that applications may temporarily become distributed across both clouds.

For example:

• Application servers may still be running in AWS.

• New application components may already be deployed in Azure.

• Databases or shared services may remain in AWS temporarily.

• DNS and routing may need to support both environments.

• Migration tools may require connectivity to source and target environments.

• Some workloads may be migrated much earlier than their dependent services.

For a long-running migration like this, what architecture would you recommend?

Some options I have been considering are:

1. Site-to-site VPN between AWS and Azure

2. Dedicated connectivity such as AWS Direct Connect + Azure ExpressRoute

3. A combination of dedicated connectivity and VPN for backup

4. A hub-and-spoke architecture in Azure with controlled connectivity back to AWS

5. Application-level connectivity rather than relying heavily on network-level connectivity

Another question is how you would handle routing.

Would you prefer to:

• Keep AWS and Azure routing domains mostly independent?

• Use a centralized hub for cross-cloud traffic?

• Advertise routes dynamically using BGP?

• Restrict cross-cloud communication to only the application dependencies that require it?

For a migration lasting several months, what approach have you found to be the most reliable and easiest to operate?

I'm particularly interested in lessons learned around:

• Routing

• DNS

• Security

• Latency

• Failover

• Monitoring

• Avoiding unexpected cross-cloud traffic costs

Would appreciate experiences from anyone who has handled a production AWS-to-Azure or Azure-to-AWS migration.