In some cases you may be an Exchange admin who doesn't have control over your AD environment. However, you still have the ability to alter the site link costs for your Exchange messaging. There are two ways to change these costs. If you change them within the Active Directory Sites and Services tool, it will be altered for both AD replication and Exchange replication traffic. If, however, you wish to make the change only for the Exchange side, you can open PowerShell and use the set-adsitelink cmdlet to create a new cost link for Exchange messaging only. You can read a full article on this here.
Now, even if you don't use Exchange (in any form, 5.5, 2000, 2003 or 2007) but you have an AD infrastructure in place, it behooves you to ensure greater efficiency, which still leads us back to the KCC, a free piece of Active Directory that may not be utilized in your environment.