Replication
in AD DS is a critical function that is necessary to fulfill the
functionality of a multimaster environment. The ability to make changes
on any domain controller in a forest and then have those changes
replicate to the other domain controllers is key. Consequently, a robust
method of distributing this information was a major consideration for
the development team at Microsoft. AD DS replication is independent of
the forest, tree, or domain structure, and it is this flexibility that
is central to AD’s success.
Sites, Site Links, and Site Link Bridgeheads
For purposes of replication, AD
DS logically organizes groups of servers into a concept known as sites.
Typically speaking, a single site should be composed of servers that are
connected to each other via high-speed connections. The links that are
established to connect two or more locations connected potentially
through slower-speed connections are known as site links. Sites are
created with site links connecting the locations together to enable the
administrator to specify the bandwidth used to replicate information
between sites.
Rather than
having information replicated immediately between servers within a
high-speed connected site, the administrator can specify to replicate
information between two sites only once per night or at a time when
network demands are low, allowing more bandwidth availability to
replicate AD DS information.
Servers that funnel intersite replication through themselves are known as site link bridgeheads.
Figure 1
shows a potential Windows Server 2008 R2 AD DS site structure. Site
links exist between offices, and a domain controller in each site acts
as the site link bridgehead. The site structure is completely
modifiable, and should roughly follow the WAN structure of an
organization. By default, only a single site is created in AD DS, and
administrators must manually create additional sites to be able to
optimize replication.
Understanding Originating Writes
Replication of objects
between domain controllers is accomplished through the use of a property
known as Originating Writes. As changes are made to an object, this
property is incrementally increased in value. A domain controller
compares its own version of this value with the one received during a
replication request. If it is lower, the change is applied;
if not, it is discarded. This simplistic approach to replication is
also extremely reliable and efficient and allows for effective object
synchronization.
