Exchange 2013/2010/2007 CAS URLs
CAS urls becomes very critical when you migrate from Exchange 2007 to Exchange 2013 and you are having 2 AD Sites with Exchange Server.
Now we are growing smarter and adapting new innovation like 3 datacenter expanded preferred architecture DAG (Kind of Geo Cluster), IP Less DAG (no CNO), Auto-reseed (Jbod and no raid), safetynet(more high available mails). Certainly we are heading to a great world. At the same time CAS connectivity planning is very crucial. So let us have a look on recommended CAS urls configuration.
When you have 2 internet facing sites then it is recommended to keep 2 different urls 1 per site so that Exchange can redirect the request to the 2nd AD site correctly. Keeping the same url will not work with Exchange 2007 because Exchange 2007 does not accept cross site redirection.
This is a site bound namespace
When we have single internet facing site then externalurl attribute in the non-internet facing site should be $null so that internet facing site can proxy to non-internet facing site.
This is a Unbound namespace
I have tried to summarize a great blog by Ross Smith IV – https://blogs.technet.microsoft.com/exchange/2015/10/06/namespace-planning-in-exchange-2016/
I was discussing unbound namespaces with Ross Smith IV and he explained that Exchange 2007 can only redirect to the local AD site so Exchange 2007 is out of question if we are planning unbound namespaces
In Exchange 2016 unbound Names Spaces will continue to work in the same way as Exchange 2013. This means Exchange 2010 and 2013 can use unbound name spaces in coexistence and 2 AD site infrastructure.
So if you have 2 AD site with Exchange 2016/2013/2010 then you can configure same External url for all.
From Exchange 2016 onwards, Microsoft is also recommending to use different internalurl than external url for Outlook anywhere and MAPI/HTTP to use different authentication mechanism in intranet (kerberos) than internet (NTLM or Basic). It is only useful if Internalurl is not resolving over public dns.
I hope this will help in clarifying more about the CAS URLs
Microsoft MVP | Exchange Server