The sites are created to physically group the computers and resources to optimize network traffic. Administrators can configure Active Directory access and replication technology to take advantage of the physical network by configuring sites. When a user logs on to the network, the authentication request searches for the domain controllers in the same site as the user. A site prevents the network traffic from traveling on slow wide area network (WAN) links.
What are Directory Tree, Directory Partition, and Replica?Directory tree is a hierarchy of objects and containers of Active Directory, which represents all the objects in the forest. Each domain controller stores a copy of a specific part of the directory tree, called a directory partition (sometimes called naming context). The copy of the directory partition is called a replica. A replica contains all attributes for each directory partition object. Each domain controller in the forest stores a replica.
What are Site Links?Site links are logical, transitive connections between two or more sites. For intersite replication to take place, site links are required to be configured. Once a site link has been configured, the knowledge consistency checker (KCC) then automatically generates the replication topology by creating the appropriate connection objects. Site links are used to determine the paths between two sites. They must be created manually. Site links are transitive in nature. For example, if Site 1 is linked with Site 2 and Site 2 is linked with Site 3, then Site 1 and Site 3 are linked transitively. The administrators can control transitivity of the site link. By default, transitivity is enabled. Site link transitivity can be enabled or disabled through a bridge.
What is Site Link Bridge?A site link bridge is created to build a transitive and logical link between two sites that do not have an explicit site link. The site link bridge is created only when the transitivity of the site link is disabled.
What is Site Link Cost?Site link cost is an attribute of a site link. Each site link has been assigned a default cost of 100. The knowledge consistency checker (KCC) uses the site link cost to determine which site links should be preferred for replication. It should be remembered that the lower the site link cost, the more preferred is the link. For example, an administrator has to configure the site link cost of links between Site 1 and Site 2. There are two site links available as shown in the image below: S1S2 is a T1 site link that uses T1 lines for replication, whereas S1S2DU uses a dial-up connection for replication. If the administrator requires that the KCC should prefer the S1S2 site link to the S1S2DU site link for replication, he will have to configure the SIS2 link with a lower cost than that of the S1S2DU link. Any site link configured with the site link cost of one (1) will always get preference over the other site links with a higher cost.
What is Bridgehead Server?A bridgehead server is a domain controller in each site, which is used as a contact point to receive and replicate data between sites. For intersite replication, KCC designates one of the domain controllers as a bridgehead server. In case the server is down, KCC designates another one from the domain controller. When a bridgehead server receives replication updates from another site, it replicates the data to the other domain controllers within its site.
What is Preferred Bridgehead Server?A preferred bridgehead server is a domain controller in a site, specified by an administrator, to act as a bridgehead server. Administrators can specify more than one preferred bridgehead server, but only one server is active at a time in a site. A preferred bridgehead server is designated to take advantage of a certain domain controller having the appropriate bandwidth to transmit and receive information. Note: If only one preferred bridgehead server is configured in a site, and it fails, then no replication will take place for that site.
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