Active Directory Site Cost Metrics in Windows 2000

In Microsoft Windows 2000, Active Directory calculates inter-site replication paths using administrator-defined site link costs and the Knowledge Consistency Checker (KCC). By running Dijkstra's shortest-path algorithm against these assigned values, the operating system determines the most efficient, lowest-cost spanning tree for directory partition synchronization across wide area networks (WANs). This mechanism prevents network saturation, optimizes bandwidth utilization, and ensures redundant failover routes between geographically separated domain controllers.

The Role of the Knowledge Consistency Checker (KCC)

The KCC is a built-in process that runs at regular 15-minute intervals on every Windows 2000 domain controller. For inter-site replication, a designated domain controller in each site—known as the Intersite Topology Generator (ISTG)—takes responsibility for evaluating site connections.

The ISTG reads the Active Directory configuration container to discover:

  • Defined sites and subnets.
  • Available site links and their transport mechanisms (IP or SMTP).
  • Assigned site link cost values.
  • Replication schedules and intervals.

How Site Cost Metrics Are Assigned and Evaluated

A site link cost is an arbitrary, unitless numerical value between 1 and 32,767 assigned by an administrator. By default, every new site link receives a cost metric of 100. Lower numbers indicate higher-preference routes, typically reflecting faster network links with greater available bandwidth and lower latency.

When multiple paths exist between two sites, the KCC evaluates the cumulative cost of each available route. It processes these metrics using Dijkstra's algorithm:

  1. Graph Construction: The KCC models the physical network as a mathematical graph, where Active Directory sites are vertices and site links are weighted edges.
  2. Path Summation: The total cost of an indirect route is the sum of the costs of all individual site links along that path. For example, if Site A connects to Site B with a cost of 100, and Site B connects to Site C with a cost of 50, the cumulative cost between Site A and Site C via Site B is 150.
  3. Route Selection: The KCC compares the cumulative metrics of all candidate paths and selects the route with the lowest aggregate value.

By default, Windows 2000 enables "Bridge all site links," treating all configured site links as transitive. Transitivity allows the KCC to route replication through intermediate sites without requiring explicit, direct point-to-point site links between every site pair.

If transitivity is disabled—often done when routed IP connectivity is not fully meshed—administrators must manually create Site Link Bridges. In this scenario, the KCC only calculates shortest paths across the explicitly bridged site links.

Tie-Breaking and Failover

If two or more paths evaluate to the exact same total metric, Windows 2000 applies deterministic tie-breaking rules. It compares the globally unique identifiers (GUIDs) or naming attributes of the server connection objects to pick a consistent path, preventing replication loops.

When a bridgehead server or link along the primary lowest-cost path becomes permanently unreachable, the KCC automatically recalculates the topology during its next cycle, building alternative connection objects across the next lowest-cost path until the preferred route is restored.