Hi All, I am planning a transition from Exchange 2003 directly to 2010. This will be a phased transition with coexistence for a time. I have several interconnected AD sites on different subnets all connected by a VPN mesh with good connectivity. I currently have one 2003 OWA server (Front End only) at Site 1, serving http/s / activesync RPC/HTTPs etc to our staff when outside of the company network. I have my CAS array planned to be at a different AD site, on a different network. I read that it states that I must install the first CAS servers on my "internet facing AD site" but is this really mandatory? I can't think of a reason why? I want to avoid having to build a new temporary OWA server in Site 2 for the legacy URL and access if I can help it. Surely the redirect for legacy access when I set the URL and add the legacy DNS in will just hit the old server regardless of which AD site it's in? If not I'll bite the bullet and build an OWA server in site 2 and decommision the old one, but hopefully not needed :-) So to give a clearer idea of plan : Site 1 : 192.168.10.0/24 - Holds OWA exchange 2003 Server, with mapping through firewall and external DNS for OWA will remain and have legacy DNS pointed at it for the coexistence period, when we are ready to move the main external DNS for owa to : Site 2: 192.168.11.0/24 - Will have New CAS(HT combined) array, 2 x Mailbox servers (DAG) Then shortly afterwards: Site 3: 192.168.12.0/24 - Will have it's own CAS array(HT role combined) and 1 x Mailbox servers to serve site 3 as primary with DAGs from/to Site 2 also as a secondary All external access for outlook anywhere will only ever be through the CAS array in site 2 where the external access to the CAS will be mapped through the firewall and the external DNS pointed to once we are ready to go live with the 2010 CAS for all requests, and we will obviously switch off and decommision owa at site 1 once all mailboxes are moved to exchange 2010. - I will install CAS roles in site 2 and 3 before moving on to adding the HT role to the same servers. Thanks, Paul.

i would consider consolidating sites if they have more than 10 MBPS of connectivity, that will reduce complexity and internet site is recomended first as the external users will connect to cas on that site that will proxy to legacy servers (backend)Dhruv

Thanks Dhruv, So it's just a recommendation that the first Exchange CAS installation is in the Internet facing AD site where my existing front end OWA server is, and it doesn't have to be there? On a different note the reason I will eventually have two CAS arrays is because I want to have a way to access email if the site where the first cas array goes entirely offline/unreachable (in reality although the first cas arrays will be on a subnet spanning multiple datacentres with redundant firewalls there's a possibility that the network goes unreachable due to human error etc). Thanks, Paul.

yes that is a recomendation. there is nothing in exchange that stops from deploying cas in non internet facing site if you go by recomndation that will for sure reduce complexity. if you upgrade the non internet facing site users who access mailbox from internet can not be moved to exchange 2010 maibbox unless you have a CAS in that site Dhruv