In Microsoft Exchange Server 2007, computers that have the Hub Transport server role installed use Active Directory sites and the costs that are assigned to the Active Directory IP site links to determine the least cost routing path from each Hub Transport server in the organization to every other Hub Transport server in the organization. After the least cost routing path is determined, the source Hub Transport servers relay messages to the target Hub Transport servers. By default, the Hub Transport servers that are located in Active Directory sites along the path between the source server and the target server do not process or relay the messages in any way.
However, you can configure an Active Directory site as a hub site so that messages are routed to the hub site to be processed before the messages are relayed to the target server. For this routing behavior to occur, the hub site must exist along the least cost routing path between two Hub Transport servers. You use the Set-AdSiteLink cmdlet to configure an Active Directory site as a hub site.
The least cost routing path is also used if a message cannot be delivered to the target site. In this case, message delivery queues at the Hub Transport servers in the closest reachable site along the least cost routing path of the target site.
By default, Exchange 2007 determines the least cost routing path by using the cost that is assigned to the Active Directory IP site link. You can use the Set-AdSiteLink cmdlet to assign an Exchange-specific cost to the Active Directory IP site link. The Exchange-specific cost is a separate attribute that is used instead of the Active Directory-assigned cost to determine the least cost routing path.
By default, Exchange 2007 does not impose a maximum message size limit on messages that are relayed between Hub Transport servers in different Active Directory sites. If you use the Set-AdSiteLink cmdlet to configure a maximum message size on an Active Directory IP site link, routing generates a non-delivery report (NDR) for any message that has a size larger than the maximum message size limit that is configured on any Active Directory site link in the least cost routing path. This configuration is useful for restricting the size of messages that are sent to remote Active Directory sites that must communicate over low-bandwidth connections.
To run the Set-AdSiteLink cmdlet, the account you use must be delegated the following:
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Exchange Organization Administrator role
For more information about permissions, delegating roles, and the rights that are required to administer Exchange 2007, see Permission Considerations.