Link state routing must be disabled whenever two or more routing groups are configured to send or receive mail from an Exchange 2007 computer that has the Hub Transport server role installed. (The Hub Transport server was formerly known as a bridgehead server). This is because Exchange 2007 uses Active Directory to determine routing topology. The Exchange 2007 servers do not propagate link state updates. If link state routing is enabled and there is more than one routing group configured to send mail to or from an Exchange 2007 Hub Transport server, routing loops might occur.