Determining the Cost

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Applies To: Windows Server 2003, Windows Server 2003 R2, Windows Server 2003 with SP1, Windows Server 2003 with SP2

You assign cost values to site links to favor inexpensive connections over expensive connections. Certain applications and services, such as Domain Controller Locator and DFS, also use cost information to locate nearest resources. Site link cost can be used to determine which domain controller is contacted by clients located in one site if the domain controller for the specified domain does not exist at that site. The client contacts the domain controller by using the site link that has the lowest cost assigned to it.

It is recommended that the cost value be defined on a site-wide basis. Cost is usually based not only on the total bandwidth of the link but also on the availability, latency, and monetary cost of the link.

To determine the costs to place on site links, perform these tasks:

  • Document the connection speed for each site link. Refer to the Geographic Locations and Communication Links worksheet for information about the connection speed that you identified. For an example of a completed Geographic Locations and Communication Links worksheet, see "Listing Communication Links and Available Bandwidth" earlier in this chapter.

    Table 3.1 lists the speeds for different types of networks.

    Table 3.1   Speeds for Different Network Types

    Network Type Speed

    Very slow

    56 kilobits per second (Kbps)

    Slow

    64 Kbps

    ISDN

    64 Kbps or 128 Kbps

    Frame relay

    Variable rate, commonly between 56 Kbps and 1.5 megabits per second (Mbps)

    T1

    1.5 Mbps

    T3

    45 Mbps

    10BaseT

    10 Mbps

    Asynchronous transfer mode (ATM)

    Variable rate, commonly between 155 Mbps and 622 Mbps

    100BaseT

    100 Mbps

    Gigabit Ethernet

    1 gigabit per second (Gbps)

  • Use Table 3.2 to calculate the cost of each site link based on WAN link speed. For WAN link speed that is not listed in the table, you can calculate a relative cost factor by dividing 1024 by the log of the available bandwith as measured in Kbps.

    Table 3.2   Calculating Costs Based on WAN Link Speed

    Available Bandwidth (Kbps) Cost

    9.6

    1042

    19.2

    798

    38.4

    644

    56

    586

    64

    567

    128

    486

    256

    425

    512

    378

    1024

    340

    2048

    309

    4096

    283

These costs do not reflect differences in reliability between network links. Set higher costs on any failure-prone network links so that you do not need to rely on those links for replication. By setting higher site links costs, you can control replication failover when a site link fails.