I'm running in circles with the namespace planning for our Exchange 2010 environment so I wanted to put it down and see if people can poke holes in any flaws that I'm missing. Setup: Two Datacenters each in their own AD Site 2 Hub/CAS servers in each site, CAS arrays being fronted by HLB 8 Databases for about 2200 email account Two mailbox databases in each Datacenter each holding a copy of a database. The sites are planning on being Active/Active so I'm thinking is that if a user's mailbox is at SiteA, both mailbox servers at SiteA will hold a current copy, SiteB mailbox1 will hold a current copy and then SiteB mailbox2's copy with be lagged. I know if we loose the WAN link SiteB will go down as it looses quorum but we've got 2 and soon a third WAN link between the sites. If a users mailbox database fails over to the other site as long as their CAS array is still functioning it should direct connect to the new mailbox server. If it's for any length of time I can update CAS array attached to that database. External users will always be connecting by VPN (no Outlook Anywhere). We will be using Juniper VPN appliances that will connect them to the OWA website if they don't have a laptop with Outlook on it. We're probably going to have Internet access at both Datacenters but I want to keep only a single namespace so that the users don't have to figure which one to use. How much of a performance site if the CAS has to proxy to another site? thanks

Hi Richard5474, Per your description, your plan seems well. I want to verify some information, do all the sites in the same forest, if so, use the single namespace would be well. About the cas service proxy and redirection, we could refer to below information: http://technet.microsoft.com/en-us/library/bb310763.aspx About the performance reason, the bandwidth of the link connection between the sites is more powerful more better. It is difficult to confirm the performace affection for one connection. I am confused that why not use outlook anywhere instead of the VPN method for the external users, in my opinion, it is better and secure feature. Regards! Gavin TechNet Subscriber Support in forum If you have any feedback on our support, please contact tngfb@microsoft.com Please remember to click Mark as Answer on the post that helps you, and to click Unmark as Answer if a marked post does not actually answer your question. This can be beneficial to other community members reading the thread.