Windows 2000 DFS Referral Ordering and Site Locality

This article explains how Microsoft Windows 2000 Distributed File System (DFS) manages referral ordering to prioritize local network resources. By integrating directly with Active Directory site topology, Windows 2000 DFS evaluates the client's IP subnet to deliver an ordered list of target servers, ensuring users connect to geographically proximate shares before attempting connections across slower wide area network (WAN) links.

Active Directory Site Integration

Windows 2000 DFS relies on Active Directory (AD) sites and subnets to establish locality. When an administrator defines sites and associates IP subnets with those sites in Active Directory Sites and Services, both domain members and domain controllers use this information to establish site awareness.

When a client queries a domain-based DFS root for a folder path, the DFS service identifies the client's IP address and maps it to a specific Active Directory site. Simultaneously, the DFS service evaluates the registered site membership of every target server hosting the requested DFS link or root replica.

The Referral Sorting Mechanism

When a Windows 2000 client accesses a link within a DFS tree, it requests a referral from the DFS server. The referral contains a list of shared network folders that satisfy the request. Windows 2000 sorts this list into two primary tiers based on site membership:

  1. In-Site Targets: Target servers that reside in the exact same Active Directory site as the requesting client are placed at the beginning of the referral list. If multiple targets exist within the same site, the DFS server randomizes their order. This randomization provides basic round-robin load balancing across local servers.
  2. Out-of-Site Targets: Any target servers that reside in sites different from the client's site are placed at the end of the referral list. Like local targets, these remote servers are also randomized among themselves.

Unlike later versions of Windows Server, Windows 2000 does not factor in inter-site link costs when building referrals. The mechanism operates on a binary basis: targets are categorized strictly as either "in-site" or "out-of-site." All non-local targets are treated as equal regardless of the physical distance or network bandwidth between the sites.

Client-Side Processing and Failover

Once the referral response reaches the client, the client caches the target list in its local Partition Knowledge Table (PKT) cache. The cache entry persists for a defined Time to Live (TTL) period, which defaults to 300 seconds for roots and 1,800 seconds for links.

The client always attempts to establish a connection with the first server in the list:

  • If the client successfully connects to an in-site server, all subsequent file operations route locally.
  • If the primary in-site target becomes unreachable, the client attempts to connect to the next server on the list.
  • If all local servers are offline or unresponsive, the client fails over to the out-of-site targets at the bottom of the list, maintaining availability across the WAN.

Through this two-tier ordering process, Windows 2000 DFS reduces cross-site traffic, minimizes latency for end users, and provides automatic failover across different physical locations.