According to Microsoft, Exchange DAG replication latency should be below 500ms roundtrip.
This DAG latency requirement has limited my deployment options in the past to very specific WAN scenarios like MPLS, Point to Point Wifi, etc. Many customers wishing to deploy a DAG across a VPN didn’t meet the latency requirements. In other words, I haven’t implemented an Exchange DAG that is replicating across the internet.
“Regardless of their geographic location relative to other DAG members, each member of the DAG must have round trip network latency no greater than 500 milliseconds between each other member. As the round trip latency between two Mailbox servers hosting copies of a database increases, the potential for replication not being up to date also increases. Regardless of the latency of the solution, customers should validate that the networks between all DAG members is capable of satisfying the data protection and availability goals of the deployment. Configurations with higher latency values may require special tuning of DAG, replication, and network parameters, such as increasing the number of databases or decreasing the number of mailboxes per database, to achieve the desired goals.”
Measuring Potential DAG Latency using Ping:
A good means of performing some initial testing on the latency of your connection is by using the Ping utility in Windows or UNIX. The ping utility will return the roundtrip network latency between you and the host that you have pinged. The response time shown in the ping results indicates the roundtrip network latency. I recommend using Ping -T in order to monitor the results over a longer period of time.