Intra versus intersite Replication

Two distinct types of replication connections exist with Active Directory sites:

1. intrasite (within sites) and
2. intersite (between sites).

An Active Directory service known as the Knowledge Consistency Checker (KCC) is responsible for automatically generating the replication connections between intrasite DCs. The KCC will create intersite connections automatically for you as well, but only when an administrator has specified that two sites should be connected via a site link. Every aspect of the KCC and the connection objects that are created is configurable, so you can control what has been automatically created and what will be automatically created via manipulation of the various options.
Note that there is a large distinction between the KCC (the process that runs every 15 minutes and creates the replication topology) and the replication process itself. The KCC is not involved in the regular work of replicating the actual data in any way. Intrasite replication along the connections created by the KCC uses a notification process to announce that changes have occurred, each domain controller is responsible for notifying its replication partners of changes. If no changes occur at all within a six-hour period, the replication process is kicked off automatically just to make sure that nothing was missed. Intersite replication, on the other hand, does not use a notification process by default. Instead, intersite replication relies on a schedule to transfer updates, using compression to reduce the total traffic size.