Example: Creating a Site Link Design
Updated: March 28, 2003
Applies To: Windows Server 2003, Windows Server 2003 R2, Windows Server 2003 with SP1, Windows Server 2003 with SP2
As an example of a site link design, Figure 3.18 shows three domain controllers from the same domain at three different Trey Research sites: Los Angeles, New York, and Seattle. The LAX-SEA and SEA-NYC site link objects are created to permit replication between domain controllers at the three sites. The site link object LAX-SEA is assigned a cost of 586, whereas the site link object SEA-NYC is assigned a lower cost of 471 because the T1 communication link that connects the Seattle and New York sites has a more available bandwidth (150 Kbps) than the 56 Kbps available bandwidth between the Seattle and Los Angeles sites.
Active Directory replication topology for the example in Figure 3.18 is computed by the KCC according to a spanning tree algorithm. For intersite replication, a spanning tree algorithm determines how to connect all the sites that need to be connected with the minimum number of connections and the least cost. For the example shown in Figure 3.18, the cost of replication between domain controller DC-2 and domain controller DC-1 is 586, the cost of replication between domain controller DC-1 and domain controller DC-3 is 471. Because no direct communication link exists between the New York and Los Angeles sites, the cost of replication between these two sites is equal to the sum of the replication costs between the Los Angeles and Seattle sites and the Seattle and New York sites (471 + 586 = 1,057).
Based on these calculated costs, the domain controllers in these three sites can replicate the domain information by creating three replication connections: DC-2 replicates with DC-1 (cost =586), DC-3 replicates with DC-1 (cost = 471), and DC-2 replicates with DC-3 (cost = 1,057). Two replication connections between DC-1 and DC-2 and DC-1 and DC-3 are sufficient for replicating the domain information between the three sites. Hence, based on the spanning tree algorithm, connection objects are created between DC-1 and DC-2, and between DC-1 and DC-3.
No site link is created between the Los Angeles and New York sites by the site topology owner because no direct communication link exists between these two locations. Even if a third site link with a cost of 1,057 is created between the Los Angeles and New York sites, no additional connection objects will be created between domain controllers DC-2 and DC-3.
With transitivity enabled in this example, DC-2 at Los Angeles always replicates with DC-1 (cost = 586) in Seattle and not DC-3 (cost = 586 + 471) in New York because the LAX-SEA site link cost is more favorable. Replication connections between DC-2 and DC-3 will only be created if DC-1 at the Seattle site is unavailable.
Figure 3.18 Example of Site Links
However, with transitivity disabled in this example, because no direct communication link exists between the Los Angeles and New York sites, DC-2 will only replicate with DC-1 and never replicate with DC-3 even if DC-1 in Seattle fails.
With site link transitivity enabled, if DC-2 in the Los Angeles site and DC-3 in the New York site are from a different domain (domain A) than DC-1 in the Seattle site (domain B), and if the LAX-SEA site link has a schedule of 18:00 hours to 24:00 hours and the SEA-NYC site link has a schedule of 17:00 hours to 20:00 hours, data can be replicated directly from DC-2 to DC-3 between 18:00 hours and 20:00 hours, which is the intersection of the LAX-SEA site link replication schedule and the SEA-NYC site link replication schedule. If the LAX-SEA site link schedule does not overlap with the SEA-NYC site link schedule, then replication between DC-2 and DC-3 can never occur.