Replication links within Active Directory can be configured to use IP, RPC or SMTP for their transport protocols. Each is explained below:
- RPC replication occurs within sites and is not compressed. As such, it occurs faster but uses more bandwidth. IP replication occurs between sites (Inter-Site Transports) and uses RPC with compression.
- SMTP replication is used for sites that cannot use the others, but as a general rule, it should never be used. It is reserved when network connections are not always available, therefore, you can schedule replication. Also, SMTP replication cannot replicate the Domain-Naming Context, the AD partition for a domain within an organization. If you are replicating the same domain, you cannot use SMTP.