Assessing Network Topology

As with the design phases discussed in the previous chapters, your first step in designing an Active Directory topology is to perform a thorough assessment of the business. Your focus during the assessment should be on the physical topology of the current network ”more specifically , the type of WAN links connecting the various business locations and the amount of bandwidth available for each one. Gathering this information helps in determining which areas to organize into Active Directory sites. Not performing an assessment at this time could result in heavy replication traffic occurring over a link that cannot support it.

WAN Links and Available Bandwidth

When assessing the WAN links connecting business locations, consider the points discussed in the following sections (the links that will be capable of supporting regular replication traffic are those considered to be fast, cheap, and reliable).

What Is the Speed of the WAN Links Connecting the Business Locations?

You need to know the speed of the links to determine whether they are fast enough to support a large amount of replication traffic. According to Microsoft, normal RPC-based replication can be performed successfully across a 19.2Kbps link, although a faster circuit is preferable. For design purposes, you should assume that a 56Kbps link is the minimum supported, even though the available bandwidth might be more important than the actual size of the link.

What Is the Utilization of the Link?

A WAN link that is fast but completely congested is far worse than a slow but unused circuit. Available bandwidth is far more important than total bandwidth.

What Is the Current Status of the Link?

If the link has been considered unreliable in the past, you will not want to have a large amount of replication traffic occurring across the link.

When you are performing the assessment of WAN links and available bandwidth, you should begin by drawing a basic diagram representing the current network topology. As you assess the various aspects of the physical topology, you can continue to add information to the diagram. Having a complete diagram will help you determine which areas to combine into sites. In addition, this can help administrators in the business understand your recommendations when a detailed diagram supports them.

After the characteristics of the WAN links in the business have been assessed, you need to assess the amount of bandwidth available over each link. This determines whether enough bandwidth is available to support replication traffic. For example, two business locations might be connected by a high-speed link, but if the link is already heavily used, it might not be capable of supporting regular replication traffic. Consider the following additional traffic-generating events over each link when assessing the amount of available bandwidth:

• How many users in each location are logging on to the network?

• When do most of the logons occur?

• Are Group Policies being applied? (Downloading Group Policies takes up bandwidth.)

• How often are users required to change their passwords?

• Is a large number of DNS queries performed throughout the day?