Lesson 7	Link multiple sites
Objective	Create site links

Link Multiple Sites

Using site links to establish cost-effective Replication Model

By associating sites with site links, you define values for your network model that provide the most cost-effective replication.
Active Directory creates a default site link named DEFAULTIPSITELINK. If you do not create new site links manually or modify the settings for the default site link, replication occurs using the default site link values. As you can see, the KCC uses specified values for cost, interval, and schedule. Let's review each of these.

Assigning Cost range Replication Value

The site link cost^[1] is an arbitrary number rather than an actual cost. You assign cost values to site links to reflect the bandwidth associated with each connection; higher cost values reflect slower connections. You can set a site link cost from 1 to 32767. By default, the cost of a site link is 100. The cost is set in the properties sheet for the <site name> object, which is located in the console tree under IP or SMTP (depending on the protocol used for the site link).

Tips for assigning site link cost

Because technologies and hardware are constantly improving, you should assign site link costs with the following in mind:

Factors to consider when assigning Site Link Costs

1) Choose proportional cost values: Ensure that the cost values that you choose are proportional to each other

2) Choose proportional cost values. For example, if the connection from Seattle to Miami is twice as fast as the connection from Seattle to Denver

3) Choose proportional cost values: you should assign a cost value to the Seattle-Denver connection that is twice the value of the Seattle-Miami connection.

4) Plan for faster connections: Allow for faster connections than those you currently have when you choose your cost values.

5) In our example, you could assign a cost of 1 to the Seattle-Miami connection and a cost of 2 to the Seattle-Denver connection.

6) Build in flexibility: Adjust costs of existing links to accomodate new ones.

7) Build in flexibility: For example, if you added a link between Miami and Denver that was slower than the Seattle-Miama connection but faster than the Miami-Denver connection, you would adjust the costs of the existing links to accomodate the new link. — 7) Build in flexibility: For example, if you added a link between Miami and Denver that was slower than the Seattle-Miami connection but faster than the Miami-Denver connection, you would adjust the costs of the existing links to accomodate the new link.

Choose proportional cost values:
If the connection from Seattle to Miami is twice as fast as the connection from Seattle to Denver
You should assign a cost value to the Seattle-Denver connection that is twice the value of the Seattle-Miami connection
Allow for faster connections than those you currently have when you choose your cost values.
You could assign a cost of 1 to the Seattle-Miami connection and a cost of 2 to the Seattle-Denver connection.
Build in flexibility: Adjust costs of existing links to accomodate new ones.
If you added a link between Miami and Denver that was slower than the Seattle-Miami connection but faster than the Miami-Denver connection

Factors To Consider when Assigning site Link Costs

Assigning an interval Replication Value

The interval setting defines the frequency of replication.
The replication interval must be at least 15 minutes and no more than 10080 minutes (or one week) in length. By default, replication occurs every three hours (an interval setting of 180).

Assigning a Schedule Replication Value

The schedule designates when the site link is available. By default, replication can occur any time.
Over a slow link, you may choose to adjust the schedule so that replication occurs only during off-peak hours.
The schedule and the interval are used together. Replication occurs only at the times allowed by the schedule, and during those times, it occurs as often as the interval allows.

This shows the replication schedule. — This shows the replication schedule

A set of site links: site link bridges

A site link bridge is an object that represents a set of site links, all of whose sites communicate with the same protocol. The Slide show below describes how the cost of a site link bridge is measured:

Cost Of SiteLink Bridges

The practical implications of an expensive site link

What are the ramifications of an expensive site link, and do administrators need to know what this cost is? A high-cost link uses more network bandwidth.
The ramifications of this may include the following:

Data transfers between the sites slow down.
Users logging on across a link may be unable to use network resources until they are logged on.
Overall performance of the network decreases.

Site links are transitive

By default, all site links are considered transitive. This means that if there is a link from A to B and a link from B to C, then A is linked implicitly to C. Site links are transitive because all site links for a given protocol implicitly belong to a single site link bridge.

Site link bridge configuration

In a fully routed IP network, no site link bridge configuration is needed. If your IP network is not fully routed, you can turn off the transitive site link feature for the IP transport, in which case you must configure site link bridges manually. In the next lesson, you will learn about monitoring replication traffic.

[1] Site link cost: A way of comparing the efficiency and desirability of using a particular site link based on speed, impact on the network and other factors.

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