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:
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:
This can be summarised as:
So, least cost really does mean least cost!
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I have 3 sites, let me call Site1 , site2 and site3.where Site 1 is holding the primary mx record for receiving the emails from external world.
Problem is when site3 losses the tunnel between Site1. All emails to be delivered to site3 are stuck in site1 itself.Instead of using alternate way that's is where site1 and site2 there is a tunnel up and from site 2 to site3.
That is the expected behaviour - if you want to route via your 2-3 network path then you would need to manually adjust the Exchange routing costs to make 2-3 the least cost path.