You might be having Exchange 2003 and planning to migrate to Exchange 2007. You might have a requirement to deploy Exchange 2007 with SCR mode at the DR Site. When migrating users from Exchange 2003 to Exchange 2007 transactions logs will be getting generated at Exchange 2003 as well as at Exchange 2007 Servers. The size of the transaction logs getting generated will depend on the size of mailboxes being migrated. If you deploy Exchange 2007 with SCR before the migration is completed, then the log shipping takes place from Exchange 2007 Mailbox server to the Exchange 2007 SCR Server. This will generate enormous traffic during the migration phase affecting the normal network traffic. It would therfore be advisable to implement SCR after the mailbox migration is completed.
Also if are going to configure SCR on a standalone server then you can provide more features to your DR Solution by adding HUB and CAS role on the same server. Please refer to the article below.
http://technet.microsoft.com/en-us/library/bb676502.aspx
Please let me know if this post was helpful.
