Active Directory Architecture: Real World Site Link Benefits & Configuration

I rarely used site links until recently, as the majority of my customers had a single site and didn’t need them. When I did need them, I was only proficient with the basics of setting up Active Directory Sites and, by extension, Active Directory Site Links.

Recently, Site Links have become a major part of my life. Seems silly, I know, but when you’re dealing with a global network with thousands of users and hundreds of servers, it’s critical to get them right and to maintain them. Site Links are definitely not fire and forget. Bandwidth and latency change over time, and it’s important to revisit them every quarter.

I’m faced with an interesting challenge because, due to uber-high security, I’m not “in the know” when it comes to how each physical site is connected to the rest of the network. I’ve been told “it’s sufficient” and little else. So, I had to figure it out on my own. More on that in a minute.

As a refresher, I want to be sure and cover what a site is exactly in Active Directory as well as define a site link.

On the surface, it’s actually very simple. In Active Directory Sites & Services, you create a subnet and then create a site, assigning that subnet to a site which is generally (but not always) related to a physical site. Once you have a site set up, you assign domain controllers to sites. All other servers and workstations then know (via the subnet) which domain controllers they should authenticate against.

Site Links are also created in Active Directory Sites & Services and provide information as to how and when the Active Directory database should be copied.

When you’re dealing with thousands and thousands of computer objects, your Active Directory database can get quite large and probably changes more often than you would think (a LOT of information that changes often is stored in Active Directory and it’s getting more robust as Microsoft transitions more and more data to Active Directory). That large database needs to be copied (often) to all other domain controllers in your domain. If you have lots of sites, you should have lots of domain controllers, and that’s a lot or replication.

Windows Active Directory does set up default site connectors, but they are worthless as they do not take into account latency or bandwidth between sites which just encourages chaos.

With Site Links though, you are able to direct that replication so that it happens when and how you want it to happen.

Back to the network I am currently working with. We don’t know what my bandwidth between sites is, and I don’t know what kind of media it rides on or how it gets there… that’s top secret info, even if we do think we need to know to do our job right.

So, we had to improvise. We simply logged on to each DC and pinged all other DC’s and recorded the latency, making a manual Visio map (did I mention we also cannot install any third party software, so we’re kind of stuck with what is built in to Windows). After that, we did tracert’s between all sites to get an idea of the how traffic was routed. Finally, we did some arbitrary data copies between DCs to get an idea of how much bandwidth we were dealing with.

It’s messy, but it works in a pinch when you can’t get details as to bandwidth, latency, media or even if you’re dealing with a carrier or a private network.

Now that we know what servers talk to each other best, we can set up our Site Links. Site Links are actually quite simple to set up, you simply specify two servers and the “cost” of that connection. The lower the cost, the more likely Windows will elect to use that link when replicating Active Directory Changes. In addition, you can specify a schedule as to when information is replicated. You don’t want it so often that it chokes the network, but you don’t want it so seldom that it presents an unacceptable security risk.

So, say you had multiple sites in the US, Japan, Germany, Guam, Columbia and some parts of Asia with lots of sand. All of those sites connect to each other with different degrees of latency and bandwidth. Germany & the US connect nice and quick, but Asia is a slow, slow link (probably satellite) to Asia, but some sites in Asia actually connect reasonably fast to one another, just not outside the country.

With a Site Link then, we can replicate changes to one domain controller in Asia, and then set up Site Links so that all other domain controllers in Asia first attempt to get their replica’s from that primary site. This allows the Active Directory architect to intelligently route what can be significant traffic along the most efficient route.

Now, multiply that times dozens and dozens of sites and you can see how even a small mistake could result in choking off parts of the network every time Active Directory replication occurs, just like clockwork.

In summary, never underestimate any part of a Windows Active Directory network or assume that “out-of-the-box” is just fine. It’s not… you really, really need to pay attention to the details in order to ensure proper operation, security and service availability.

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