Information Technology Reference
In-Depth Information
The Cost field is an administrator-assigned value that represents the bandwidth of the connec-
tion between sites. The default value is 100. An administrator can alter this value to influence
which path is chosen when more than one path exists between two sites. As shown in Figure 4-21,
Site A replicates with Site B and Site C through the corresponding site links, but Site A has two
options for replicating with Site D: the link with Site B or the link with Site C. The site link cost
determines that Site A will use the link with Site B. Site link costs are additive, so the total cost
for Site A to replicate with Site D through Site C is 400; the total cost to replicate with Site D
via Site B is only 300. When you have more than one path option between two sites, the lower
cost path is always used unless links in the path become unavailable. In this case, the replication
process reconfigures itself to use the next lower cost path, if available. Site links are transitive by
default, which means Site A can replicate directly with Site D, and Site C can replicate directly
with Site B, without creating an explicit link between the two sites.
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Site link cost 200
Site A
Site B
Site link cost 200
Site link cost 100
Site link cost 200
Site C
Site D
Figure 4-21
Site replication topology
Bridgehead Servers
You learned that intrasite replication occurs among several domain
controllers after the KCC creates the topology. Intersite replication occurs between bridgehead
servers. When the KCC detects that replication must occur between sites, one domain controller
in each site is designated as the Inter-Site Topology Generator (ISTG). The ISTG then designates
a bridgehead server to handle replication for each directory partition. Because bridgehead servers
perform such a vital function in multisite networks, and this function can consume considerable
server resources, the administrator can override automatic assignment of a bridgehead server and
assign the role to a specific domain controller. Configuration of bridgehead servers is discussed
in Chapter 10.
Active Directory is based on the X.500 and LDAP standards, which are standard protocols
for defining, storing, and accessing directory service objects.
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OUs, the building blocks of the Active Directory structure in a domain, can be designed to
mirror a company's organizational chart. Delegation of control can be used to give certain
users some management authority in an OU. You need to be familiar with OU permissions
and permission inheritance to understand delegation of control.
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Large organizations might require multiple domains, trees, and forests. Some terms for
describing the Active Directory structure include directory partitions, operations master
roles, Active Directory replication, and trust relationships.
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