Compare replication within a site and between sites
Define the differences between replication within a site and between sites.
Using Connection Sites for Intrasite/Intersite Replication
Connection objects establish the path for replication between domain controllers within a site (intrasite replication) and between domain controllers in different sites (intersite replication). However, you must consider important differences between these types of replication when creating sites and placing domain controllers within those sites.
Intra-site replication: Replication of the Active Directory partition that occurs within a site.
Intersite replication: Replication of the Active Directory partition that occurs between different sites.
Replication within Active Directory Site: Intrasite Replication
It is important to remember that replication within a site is designed to work with fast, reliable connections. Intrasite replication happens through a process called change notification.
Intrasite Replication through Change Notification
To keep bandwidth usage down, replication within a site occurs through a change notification process.
The series of diagrams below demonstrates how this happens:
When a change to an object occurs on a domain controller, the domain controller waits for a configurable interval.
When the interval has passed, the domain controller sends a notification message to its replication partners, informing them of the change.
The domain controller continues to accept changes during this interval.
When the replication partners receive the change notification, they copy the changes from the originating domain controller.
If no changes occur during a configurable period (6 hours by default)
A domain controller initiates a replication sequence with its replication partners to ensure that no changes were missed.
Consider the available bandwidth for uncompressed replication traffic
Because a site assumes fast, highly reliable links, replication traffic within a site is uncompressed. This helps reduce the processing load on the domain controllers.However, this uncompressed traffic can increase the network bandwidth required for replication messages. This is why it is important that you determine the actual available bandwidth on your links when determining the site layout that your network requires. Remember to consider the available bandwidth for uncompressed replication traffic when configuring sites for your network.
Certain security-sensitive updates are replicated immediately within a site. The following are flagged for security-sensitive update:
Changes to the account lockout policy
Changes to the domain password policy
Changes to the password on a computer account
Replication of a newly locked-out account
Changes to a Local Security Authority (LSA) secret
Replication between sites: Intersite Replication
In contrast to replication within a site, replication between sites is designed under the assumption that the network links between sites have limited available bandwidth and may not be reliable. Understanding how replication between sites occurs is critical when determining how to create a multiple site structure.
Intersite Replication through Scheduling
Replication between sites does not occur through a change notification process. When and how often replication occurs between sites are defined by configurable values, such as a schedule and an interval. The schedule determines at what times replication is allowed to occur, and the interval specifies how often domain controllers check for changes during the time that replication is allowed to occur.
Consider the Processing load of Compressed Traffic
Replication traffic between sites is designed to optimize bandwidth. This is accomplished by compressing all replication traffic between sites. Replication traffic is compressed to 10-15 percent of its original size before it is transmitted. Although compression optimizes network bandwidth, it imposes an additional processing load on domain controllers. Urgent replication is not available for replication between sites.
Replication at a Glance
What determines the success of each of these types of replication? The table below examines the process, goals, and factors influencing the success of each:
Success factor depends on balance between goal and cost
Fast, reliable connections
Change notification and replication sequence
Cost: increased network bandwidth
Success factor: monitoring the increased load on the network