Building Site Link Bridges

Youve divided your Windows 2000 Active Directory into sites to reduce network traffic and free up bandwidth. Now youll need to implement site links and site link bridges to achieve replication between sites.

By Brien M. Posey | Posted Oct 30, 2000
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In the first two parts of this series ( Using Sites in Windows 2000 and Inter-site Replication ), I've explained how breaking your Active Directory into sites can reduce replication-related network traffic over slow WAN links. Part 1 explains the basic workings of Active Directory sites, and Part 2 focuses on the specifics of site links. In this article, I'll continue the discussion by talking about site link bridges and some other ways of achieving Active Directory replication between sites.

Working With Multiple Sites

As you may recall from part 2, simply dividing your Active Directory into sites isn't enough. If you want the sites to exchange Active Directory information, you must implement a site link . A site link tells Windows 2000 which sites should be replicated, and how often that replication should occur. In Part 2, I show you how to build a site link between two sites. However, in real life, networks that are big enough to be broken into sites tend to use more than two sites. When you start working with more than two sites, you run into some interesting situations.

In Windows 2000, any time you link more than two sites using the same link transport (IP or SMTP), those sites are said to be bridged. Of course, this is assuming that the site links involve common sites. For example, in Figure 1, you can see three sites linked by common IP-based site links. The sites in the figure use the names site A, site B, and site C. Because these sites share common site links, each site can communicate directly with any other site, in much the same way that nodes on an IP-based network can communicate directly with each other.

Figure 1
Figure 1: Sites sharing a common link can communicate directly with each other.

To put this concept into more precise terms, when sites are bridged, they are said to be transitive in nature. This means that if you create a site link, any sites that fall into that site link are bridged automatically, and can therefore communicate directly with each other. If your entire organization is composed of sites that are linked by a single common site link, then those sites are automatically bridged, and are therefore transitive in nature.

As you can see, Windows 2000 was designed in a way that was intended to save you work whenever possible. After all, in Part 2 I linked some sites together and the topic of a site link bridge never even came up. Assuming that your sites all exist on a fully routed IP network, you'll never need to manually create a site link bridge.

However, things are rarely this simple in the real world. For example, in networks that consist of several isolated IP segments or a combination of IP and SMTP site links, the isolated or dissimilar portions of the network wouldn't have site link bridges between them by default. In such situations, it's necessary to manually create site link bridges if you want to enable replication between the various sites.

Manually Creating Site Link Bridges

A site link bridge is a logical device that connects existing site links. For example, suppose that you have two dissimilar networks. The first network contains sites A and B and the second network includes sites C and D. The site link within each of the two networks establishes the transitive link among the sites within the network. However, because the two networks are isolated, the sites can't replicate Active Directory information between the two networks. This is where the site link bridge comes in: It connects the two networks at the site link level. To better understand the idea of a site link bridge, consider that it links site links together similarly to the way that site links link sites together.

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