Active Directory Replication

Another potential problem in a multi master replication model would be the possibility of replication loops occurring. Quite simply, A might let B and C know that changes exist. After receiving the changes, B might also try to send notification to C, who has already received them. To accomplish this, Active Directory uses a technique called propagation dampening. This technique has every domain controller hold a table in memory called the up-to-dateness table, which stores update sequence numbers (USNs) for every domain controller. When a domain controller handles an originating update, it updates its USN number, and this information is held on all other domain controllers. For example, if an originating change was made on DC1, and the USN on DC1 increased to 2667, all domain controllers would have this information in their up-to-dateness table, and would not require it to be replicated again from other partners if they offered the same updates.

Replication between sites works differently than replication within sites in an Active Directory environment. AS mentioned early, a site is a collection of high-speed subnets. In order to define sites and subnets, the proper objects must be created and associated. Usually you would start by creating site objects, and then associating subnet objects with sites. I have created 3 new sites, and associated 3 subnets with the site called Toronto.

In order to control replication between sites, we need to link the sites together using site links. A site link connects the 2 or more sites for the purpose of creating a pathway for replication. Once a site link has been created, properties can be set on that link, including the cost, schedule, and interval. The cost is a number between 1 and 32767 that helps determine the links that will be crossed in the event that multiple paths exist. The lower the cost number, the higher the priority of the path. Usually you map costs to speed of links – maybe 50 for a T1 link, 500 for a 56K link, and so forth. The schedule defines when replication is allowed to happen. By default this is always, but it could be configured to only allow replication at night, for example. The interval controls how often replication can occur between sites. By default this is set to every 180 minutes, but it can be set to lower or higher values if you choose. Note that inter-site replication does not use change notification. Instead it uses the schedule and interval values to figure out when replication occurs. This is very different than it NT 4, when change notification was used throughout the environment.

Author: Dan DiNicolo

Dan DiNicolo is a freelance author, consultant, trainer, and the managing editor of He is the author of the CCNA Study Guide found on this site, as well as many books including the PC Magazine titles Windows XP Security Solutions and Windows Vista Security Solutions. Click here to contact Dan.