Microsoft: Intrasite and Intersite Replication of Active Directory

There are two types of Active Directory replication based on site topology. Intrasite and Intersite replication. In intrasite replication, all the domain controllers inside the same site will replicate each other. In Intersite replication, Selected Domain controllers of two different sites will replicate during specified interval. Domain controller which is assigned for replication over the site is called Bridge Head Servers.

<< Interval for Intrasite Replication >>
Intrasite replication occurs automatically on the basis of change notification. Intrasite replication begins when you make a directory update on a domain controller. By default, the source domain controller waits 15 seconds and then sends an update notification to its closest replication partner. If the source domain controller has more than one replication partner, subsequent notifications go out by default at 3 second intervals to each partner. After receiving notification of a change, a partner domain controller sends a directory update request to the source domain controller. The source domain controller responds to the request with a replication operation. The 3 second notification interval prevents the source domain controller from being overwhelmed with simultaneous update requests from its replication partners.

But for some of the directory updates, domain controllers will not wait for 15 seconds for replication. This situation is called Urgent Replication. Some of the directory updates such as assigning of account lockouts and changes in the account lockout policy, the domain password policy, or the password on a domain controller account etc. are example for Urgent Replication.

<< Interval for Intersite Replication >>
Intersite replication occurs between replication partners in two different sites. Active Directory preserves bandwidth between sites by minimizing the frequency of replication and by allowing you to schedule the availability of site links for replication. By default, intersite replication across each site link occurs every 180 minutes that is 3 hours. You can modify this replication interval, and it can be brought down till 15 minutes. But its always recommended to keep the default interval because the intersite replication occurs between low speed WAN links, hence reducing the replication interval could cause high network traffic and latency.