One of the common issue when you are transition from Exchange 2003 to Exchange 2010 in multi Ad site environment that there is no Routing group connector created for each of the AD site. Exchange 2010 would create one routing group connector in the Ad site during the installation of first Hub transport server in the environment. This might not happen in the subsequent installation of Hub servers in multiple AD sites where Exchange 2003 servers is also residing.
To avoid this scenario we need to create Routing group connector manually for each of the AD site. Below is the example to create new Routing Group connector for a particular AD site. We need to make sure we add multiple exchange 2010 hub server for the parameter SourceTransportServers and similarly multiple exchanges 2003 at parameter TargetTransportServers. These servers should be based on the particular AD site and it will not allow adding servers form multiple AD site. It’s also important to make sure Bidirectional is set to $true. This is because routing group connectors are unidirectional by default and it has to be enable bidirectional if it needs to route email both the side else we need create two routing group connector in each site and swap the values of sourcetransportserver and Targettransportservers parameter. PublicFolderReferralsEnabled parameter would help public folder referrals to use routing group connectors. This is important if you want to replication public folder between exchange 2003 and exchange 2010
Below is the command to create new routing group connector for a specific AD site
New-RoutingGroupConnector -Name "connector name – AD site" -SourceTransportServers "Hub2010-01.contoso.com, Hub2010-02.contoso.com” -TargetTransportServers "Exch2003-01.contoso.com, Exch2003-02.contoso.com” -Cost 10 -Bidirectional $true -PublicFolderReferralsEnabled $true
As multiple routes exists in the organization between exchange 2003 and exchange 2010, this can cause looping. To avoid looping we need to suppress Link state updates. Below TechNet link has the registry update steps to suppress Link state updates