Site Link Costs
AD uses the Knowledge Consistency Checker (KCC) to make decisions about replication. It does this by looking at costs assigned to site links. Costing is an inverse weighting system. The higher the cumulative cost, the less likely the KCC will use that replication path (or inversly, the lower the cost, the more likely it is to use that path). You should assign site link costs using the following formula:
1024/log(x)
Where x = Link Speed in kbps. For example, link cost for a T1 (1.5mbps or 1536 kbps) would look like this:
1024/log(1536) = 321 (321.366 rounded down)
Intra-Site Replication
Inter site replication works as follows, when a DC receives an update, it waits 15 sec, then sends out a "change notification" to its replication partner, if it has more than one replication partner, it waits an additional 3 sec, then sends a change notification to subsequent partners (at 3 sec intervals). When a DC received a change notification, it sends a "directory update request" to the source DC.
https://technet.microsoft.com/en-us/library/cc728010(v=ws.10).aspx
Force (near) Instant Replication
- Open AD Sites and Services
- Select a IP Site Link (Under Inter-Site Transports)
- Right Click -> Properties -> Attribute Editor
- Change 'options' attribute to 1 (USE_NOTIFY)
This will cause replication across this site link to happen almost immediately (Like when DC's are in the same site, approximately 15 sec). Essentially when a DC receives a change, it broadcasts a change notification to the other DC, which then replicates the changes as soon as the notification is received.
Only for fast / low latency site links!
References
http://www.mdmarra.com/2013/07/making-active-directory-replication.html