Intrasite replication in Active Directory takes place between domain controllers within the same site. This makes intrasite replication an uncomplicated process. When changes are made to the replica of Active Directory on one particular domain controller, the domain controller contacts the remainder of the domain controllers within the site. The domain controller checks the information it contains against information hosted by the other domain controllers. To perform this analysis, the domain controller utilizes logical sequence numbers. Intrasite replication utilizes the Remote Procedure Call (RPC) protocol to convey replication data over fast, reliable network connections. With intrasite replication, replication data is not compressed.
Intersite replication takes place between sites. Intersite replication can utilize either RPC over IP or SMTP to convey replication data. This type of replication has to be manually configured. Intersite replication occurs between two domain controllers that are called bridgeheads or bridgehead servers. The role of a bridgehead server (BS) is assigned to at least one domain controller in a site. A BS in one site deals with replicating changes with other BSs in different sites. You can configure multiple bridgehead servers in a site. It is only these BSs that replicate data with domain controllers in different domains by performing intersite replication with its BS partners. With intersite replication, packets are compressed to save bandwidth. This places additional CPU load on domain controllers assigned the BS role. BSs should therefore be machines that have enough speed and processors to perform replication. Intersite replication takes place over site links by a polling method which is every 180 minutes by default.