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Active Directory Site Link Costs

Friday, October 28, 2011

When designing Active Directory Site Links, the first step is to determine the site link's cost, schedule, and replication interval.  In this summary, we will focus on the cost attribute.  Determining the cost associated with the replication path is required because the KCC uses cost information to determine the least "expensive" route for replication between two sites that replicate the same directory partition.

The default cost for a site link is 100.  However, it is more appropriate to assign a cost link relative to the available bandwidth of the network connection between the two sites replicating.  The lower the cost, the more preferred the connection becomes.  Here is an example of some costs (recommended by Microsoft) as a best practice.

Available BandwidthCost
9.6 Kbps1042
19.2 Kbps798
38.4 Kbps644
56 Kbps586
64 Kbps567
128 Kbps486
256 Kbps425
512 Kbps378
1024 Kbps340
2048 Kbps309
4096 Kbps283

For network link speeds that are not listed in the table, you can calculate the relative cost factor by dividing 1,024 by the log of the available bandwidth, as measured in Kbps.  For example, if the available bandwidth is 8192 Kbps, you would use the following formula --> 1024 / log(8192) = 262 (round to the nearest whole number)

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