Exchange 2007 & 2010 use a different message routing design than Exchange 2003.  This is an important aspect to understand when transitioning from Exchange 2003 upwards due to the change in behaviour.  

TechNet has articles that discuses these concepts :

Exchange 2010 http://technet.microsoft.com/en-us/library/aa998825.aspx 

Exchange 2007 http://technet.microsoft.com/en-us/library/aa998825(EXCHG.80).aspx 

Exchange 2003 http://technet.microsoft.com/en-us/library/aa998800(EXCHG.65).aspx

Exchange 2007 and 2010 base their routing topology off the defined AD site design, and do not carry forward the Exchange 2000/2003 concept of Routing Groups (though mail can still be delivered to servers in the older Routing Groups during a transition).

When sending a message from one Exchange site to another, the Hub will determine the least cost route and only use this one route.  This means:

• A single route will be picked and then it is used. 
• No other routes will be used
• Do not take connector availability into consideration
• Least cost routing really is least cost routing

Determining the least cost route can be easily determined if you have site link costs that sum up to different totals.  But should you design an Exchange 2007 environment that has two paths between sites that has the same cumulative site cost it may lead you to think that both connectors will be used in a load balancing scenario.  Still in this case a single least cost path will be determined.  Having two paths with the same cost does NOT mean that both connections will be used!  Least cost routing really does mean least cost, i.e.  use the one that has the lowest cost and only that least cost route.  let’s dig into this a little. 

Here is an example of an Exchange organisation that has 5 sites.  The relevant cost of the link is show on the respective segment. 

Now, let’s review three examples to see how the message path is determined:

Example 1 A message that is being relayed from Site A to Site D can follow two possible routing paths: Site A-Site B-Site D and Site A-Site C-Site D. The costs assigned to the IP site links in each routing path are added to determine the total cost to route the message. In this example, the routing path Site A-Site B-Site D has an aggregate cost of 20. The routing path Site A-Site C-Site D has an aggregate cost of 10. Routing selects path Site A-Site C-Site D.

Example 2 A message is being relayed from Site B to Site D. There are three possible routing paths: Site B-Site D with a cost of 15, Site B-Site E-Site C-Site D with a cost of 15, and Site B-Site A-Site C-Site D with a cost of 15. Because more than one routing path results in the same cost, routing selects the routing path Site B-Site D. This has the least number of hops.

Example 3 A message is being relayed from Site A to Site E. There are two possible routing paths: Site A-Site B-Site E with a cost of 10, and Site A-Site C-Site E with a cost of ten. Both routing paths have the same cost and same number of hops. The alphanumeric order of the Active Directory sites immediately before Site E is compared. Site B has a lower alphanumeric value than Site C. Therefore, routing selects the routing path Site A-Site B-Site E.

After the least cost routing path has been determined, Exchange 2007 & 2010 routing does not consider alternative routing paths.

Why did Example 3 behave the way that it did?  Multiple factors come into choosing the least cost path which include:

• Linked connectors If the Receive connector that the message is received on is linked to a Send connector, messages are routed to that Send connector regardless of cost. This configuration always takes precedence.
• The cost assigned to the IP site links and routing group connectors that must be traversed to reach the destination If more than one routing path exists between a source server and a destination server, the routing path with the lowest aggregate cost is selected.
• The name assigned to an Active Directory site If more than one routing path results in the same aggregate cost, the routing component makes an alphanumeric comparison of the name of the Active Directory sites that precede the target site along each routing path. And the routing path where the Active Directory site nearest to the destination is lowest in alphanumeric order is used.
• The name assigned to a routing group connector If more than one routing path results in the same aggregate cost, the routing component makes an alphanumeric comparison of the name of the routing group connectors that come before the target destination along each routing path. The routing path where the routing group connector nearest to the destination is lowest in alphanumeric order is used.
• The address space assigned to a Send connector The Send connector with the most specific address space match to the destination is selected.
• The cost assigned to the address space configured on a Send connector If more than one Send connector is assigned the same address space, the routing component compares the cost assigned to the address space. The Send connector with the lowest cost is selected.
• The connector state The Exchange 2007 routing component only considers enabled connectors when it calculates the routing path. However, earlier versions of Exchange Server do not consider connector state. For more information, see Message Routing in a Coexistence Environment.
• Connector scope A connector may be limited to use by Exchange 2007 servers that are located in the same Active Directory site as the source transport servers for the connector. In earlier versions of Exchange Server, the connector scope could be limited to servers that have the same routing group membership.
• Message size restrictions The message size constraint specified on a connector must be larger than the size of the message being routed. Connectors with a message size restriction that is less than the size of the message are eliminated from routing consideration.
• The proximity of the destination to the sending server Routing will prefer the server that is closest, in this order: local server, server in the same Active Directory site, server in a remote Active Directory site or routing group.

This can be summarised as:

• First, calculate the least cost routing path by adding the cost of the IP site links and of any routing group connectors that must be traversed to reach the destination. If the destination is a connector, the cost assigned to the address space is added to the cost to reach the selected connector. If multiple routing paths are possible, only the routing path with the lowest aggregate cost is used.
• If more than one routing path has the same aggregate cost, the number of hops in each path is evaluated and the routing path with the least number of hops is used.
• If more than one routing path is still available, the name assigned to the Active Directory sites or routing group connectors before the destination are considered. The routing path where the Active Directory site nearest to the destination is lowest in alphanumeric order is used. If the site nearest to the destination is the same for all routing paths being evaluated, an earlier site name is considered.

So, least cost really does mean least cost!

Cheers,

Rhoderick