What are Site Link Bridges in Active Directory

In Active Directory, a site link bridge is a logical connection between two or more site links that enables domain controllers in different sites to replicate directory data with each other, even if the sites are not directly connected by a site link. Site link bridges are used to replicate directory data between sites in a multiple-site network when there are multiple hops or intermediaries between the sites.
For example, consider a network with three sites (Site A, Site B, and Site C) and two site links (Site A-Site B and Site B-Site C). Without a site link bridge, domain controllers in Site A and Site C would not be able to replicate directory data with each other directly. However, if you create a site link bridge between the two site links, the domain controllers in Site A and Site C will be able to replicate directory data with each other through Site B.

To configure a site link bridge in Active Directory, follow these steps:

1. Open the Active Directory Sites and Services console. You can do this by going to Start > Administrative Tools > Active Directory Sites and Services.
2. In the console tree, expand the Sites container and then select the Inter-Site Transports folder.
3. Right-click the folder and select New Site Link Bridge.
4. In the New Object, Site Link Bridge dialog box, enter a name for the site link bridge and select the site links that you want to include in the bridge.
5. Click OK to create the site link bridge.
6. To modify the properties of the site link bridge, right-click the site link bridge and select Properties.
7. In the Properties dialog box, you can modify the following settings:
   
   1. Replication Frequency: This setting determines how often replication between the sites will occur.
   2. Replication Schedule: This setting determines the specific times when replication between the sites will occur.
   3. Cost: This setting determines the relative cost of using the site link bridge for replication. Higher cost values are used for slower or more expensive links.
   4. Transport: This setting determines the transport protocol that will be used for replication over the site link bridge. The default transport is the Remote Procedure Call (RPC) over IP.
8. Click OK to save your changes.

In summary, a site link bridge in Active Directory is a logical connection between two or more site links that enables domain controllers in different sites to replicate directory data with each other, even if the sites are not directly connected by a site link. Configuring a site link bridge involves creating the bridge and modifying the properties of the bridge to control the frequency, schedule, cost, and transport protocol for replication between the sites.