As I mentioned earlier, Dfs is a site-aware service, which means that when a client accesses a link and multiple link targets are available for that link, Dfs first attempts to redirect the client to a link target in the client's local site. If Dfs can't find an available local target, it redirects the client to another, randomly selected, link target. Dfs's redirection strategy reduces the use of WAN bandwidth when a local link target is available.
Dfs's default behavior of redirecting a client to any alternative link target when it can't find a local link target can be inefficient. For example, if Dfs can't find a local target in Dallas, it might redirect the client to a target in London, although another link exists in New Orleans over a faster connection. However, you can adjust your Dfs configuration so that Dfs redirects clients more efficiently. One option is to configure Dfs so that it redirects clients only to link targets in the users' local sites. You enable this option—called Restricted Same-site Target Selection mode—by executing the Dfsutil command and specifying the /insite parameter, like this:
dfsutil /root:\\<server> \<dfs root> /insite /enable
The downside of Restricted Same-site Target Selection is that when Dfs can't find a local target, users simply can't access the resource.
Alternatively, if you're running Windows 2003 on your DCs and Dfs servers, you can enable Least-Expensive Target Selection mode. In this mode, if a local-site link target is unavailable, Dfs redirects the client to a link target that's the lowest cost in terms of the amount of bandwidth Dfs uses to access the AD site-link definitions. Least-Expensive Target Selection mode minimizes the usage of slower links and lets clients access shares more quickly. To enable Least-Expensive Target Selection, run the command
dfsutil /root:\\<server> \<dfs root> /sitecosting /enable