MCSA/MCSE Self-Paced Training Kit (Exam 70-270): Installing ...

14-10 Chapter 14 Overview of Active Directory Service
■ Domain controllers detect collisions, which can occur when an attribute is modified
on a domain controller before a change to the same attribute on another
domain controller is completely propagated. Collisions are detected by comparing
each attribute’s property version number, a number specific to an attribute that is
initialized on creation of the attribute. Active Directory resolves the collision by
replicating the changed attribute with the higher property version number.
■ Having more than one domain controller in a domain provides fault tolerance. If
one domain controller is offline, another domain controller can provide all
required functions, such as recording changes to Active Directory.
■ Domain controllers manage all aspects of user domain interaction, such as locating
Active Directory objects and validating user logon attempts.
In general, there should be at least one domain controller for each domain in each site
for authentication purposes. However, authentication requirements for your organization
determine the number of domain controllers and their locations.
Sites
A site is a combination of one or more Internet Protocol (IP) subnets connected by a
highly reliable, fast link to localize as much network traffic as possible. Typically, a site
has the same boundaries as a local area network (LAN). When you group subnets on
your network, you should combine only those subnets that have fast, cheap, and reliable
network connections with one another. Fast network connections are at least 512
kilobits per second (Kbps). An available bandwidth of 128 Kbps and higher is sufficient.
With Active Directory, sites are not part of the namespace. When you browse the logical
namespace, you see computers and users grouped into domains and OUs, not
sites. Sites contain only computer objects and connection objects used to configure
replication between sites.
Note A single domain can span multiple geographical sites, and a single site can include
user accounts and computers belonging to multiple domains.
On the CD At this point, you should view the multimedia presentation “The Physical Structure
of Active Directory,” which is included in the Multimedia folder on the CD accompanying
this book.