Sites and Services distinguishes between two types of replication:

  • Intrasite replication occurs between domain controllers within a site. Intrasite replication is not compressed and happens automatically between all domain controllers within the site. You can modify the frequency to occur up to four times per hour.
  • Intersite replication occurs between bridgehead servers between sites. Intersite replication is compressed, scheduled, and configured to use a specific networking protocol. Compressing replication data allows the data to be transferred over WAN links more quickly, thereby conserving network bandwidth. To customize intersite replication, configure sites and site links.

Replication uses one of following transport protocols:

Protocol Description
Directory Services Remote Procedure Call (DS-RPC) Directory Services Remote Procedure Call (DS-RPC), also known as IP in Active Directory Sites and Services, is used for intra-site and inter-site replication.
  • Remote Procedure Calls (RPC) runs over IP.
  • IP replication adheres to replication schedules by default, although you may configure Active Directory replication to ignore schedules.
  • IP replication does not require a Certification Authority (CA).

Note: By default, both intrasite and intersite transport for AD DS replication is RPC over IP.

Inter-Site Messaging—Simple Mail Transfer Protocol (ISM-SMTP) Inter-Site Messaging—Simple Mail Transfer Protocol (ISM-SMTP), also known as SMTP in Active Directory Sites and Services, allows replication within mail messages in environments where wide area network (WAN) links are not available. In this case, replication occurs according to the messaging schedule and not the site link schedule. SMTP replication:
  • Is used for replication over site links (inter-site). Is not used for replication within a site (intra-site).
  • Uses 56-bit encryption.
  • Is used for high latency links where RPC over IP replication would probably fail.
  • Can replicate only the configuration and schema directory partitions and global catalog read-only replicas (not writable domain data).
  • Requires an enterprise CA when you use it over site links.