You may get the following error message when trying to replicate across a firewall:

Operation timed out. The configuration server may be busy or down.

When setting up replication across some firewalls, if the packet size is not set to 1400 bytes or 1 KB, you may encounter some errors. For example, when performing the Automatic Synchronization operation or changing the packet size you may see this message.

First check the firewall settings and the logs to verify if the packets are dropped, because the packet size exceeds the required value.

To fix the problem you may want to delete the RDS and recreate it. However, before doing so you must ensure that the firewall configuration is completed as required and the necessary ports have been opened.

To avoid such problems, when creating an RDS using the wizard, set the packet size to 1KB or 1400 bytes (default). If you still face the problem set the packet size to 1300 bytes.