Almost all are aware of the fact that Exchange 2010 uses AD Sites and Services information to route emails from one AD site to another. All site links will have a default AD cost of 100. Smaller companies might leave the configuration as it is, as they will only have a single site in most cases.
Larger organizations will have fairly good number of AD sites and the AD site links will be configured with the appropriate AD cost for the links. By default, Exchange 2010 uses the link which has the least AD cost to send emails from one AD site to another. You may come across organizations where the AD costs will have to be altered for an efficient email routing, but you won’t be given the go ahead. This mostly happens in large organizations, as changing the existing AD site costs may affect the production network or the AD team just don’t want to make any changes.
Exchange 2010 (infact 2007 as well) has a solution for the above scenario. There is a way by which you can change the email routing without making any changes to the AD site link costs. It is known as “Exchange Cost” and is not configured by default.
Once an exchange cost for a site link is specified, it overrides the AD link costs for email routing. I have to stress that it only overrides the email routing topology. I have three sites in my example and in order to change the routing topology, I ran the command below.
Set-ADSiteLink –identity “link name” –ExchangeCost “cost”
Running the above command changed my email routing for India – London to go via Dubai site, rather than taking the direct link. Looking at the AD cost, emails should take the direct route, but it doesn’t. Once the exchange cost is configured, it overrides the AD site link costs for email routing. This makes the email to flow through the Dubai site in my example.
The maximum message size that passes through a site link can be restricted as well. It will come in handy if the link between the site is not great. Run the command below to specify a size limit for communications between various AD sites.
Set-ADSiteLink –identity “link name” –MaxMessageSize “size”
You can now change the email routing without involving the AD guys!