How to force Active Directory Replication between Domain controllers?

I believe it's Active Directory Sites and Services, but I could be wrong on that one.  basically, you want to right-click on your domain (from one of those AD tools) and hit "Replicate Now".

Replication between domain controllers makes sure that both domain controllers have identical copies of your active directory information (active users, group policies, permissions, etc).  The whole reason for having multiple domain controllers for a single domain is so that both servers can authenticate users as they log on/off or access network resources.  If one dies, the other can take over.  Also, when both are up, they can tag-team the workload.

A common scenario is two domain controllers at different sites.  Since the connection between the two site is likely to be slow, you want to keep as much traffic as possible from crossing over those links.  If you are at site A and you log on to the network, you would want a domain controller on site A to authenticate you, rather than using the slow (and potentially unreliable) connection between to have the controller in site B authenticate you.

Since the link between the two sites could go down at any time, both domain controllers need exactly identical copies of the domain information.  If a worker in site B gets fired, and you are in site A, you must disable that person's account.  Domain controller A must then replicate that information to domain controller B so that person can't log on to his computer anymore.

Hello
Your domain controllers are databases that hold security and configuration related info about your computers, user accounts, groups and such.  Since a network may have more than one domain controller, there needs to be a way to synchronize data between them.  If you change Bob's password on DC1 but Bob gets logged on to the network by DC5, there has to be some way for DC1 to exchange its data with DC5 and that method is replication.  Usually replication happens quickly (on a LAN) and on a set schedule on a Wan.  If you change a password or something and you want all the local (LAN) DCs to get the change, this *should* happen by default and it should happen quickly.  Across the WAN, this will take longer and it is possible to have replication occur only once a night, for example.

To manually trigger replication, one way is to go to Active Directory Sites and Services.  Then expand Site, Default-First-Site, Servers, <name-of-DC-you-want-to-trigger>, click NTDS Settings, click the + symbol to the left of NTDS Settings and then on the *right hand pane of the screen* find the object with the text <automatically generated>, right-click and choose "replicate now".