Legacy Namespace for Exchange 2007 to 2013 co-existence

Question

We are migrating from Exchange 2007 to 2013, during the co-existence phase, where is the legacy.{domain.com} namespace used? We are at the point now that we want to move all services over to the Exchange 2013 CAS servers, however... GPO settings
are used to point outlook clients to mail.{domain.com} for Outlook Anywhere. If DNS is updated to point mail.{domain.com} to the Exchange 2013 servers, will there be an issue with connectivity for people still on the Exchange 2007 servers? Do these people
need to point to legacy.{Domain.com} or will mail.{domain.com} proxy the connection to the legacy namespace? I would like to know if the GPO settings will interfer with the settings that Autodiscovery provide back.
I have read a bunch or articles on the approach, but I am still fuzzy on where legacy.{domain.com} comes into play.
Thanks in advance for your help.

Answer 1

In coexistence with exchange 2013 and legacy version the request happens in 2 types.
For Exchange 2010 –
Exchange 2013 does a Proxy for owa and ews requests for users in exchange 2010.
For Exchange 2007 –
Exchange 2013 does redirection for owa and ews requests for users in Exchange 2007.
Certificates:
All the required SAN entries for UM,webservices and activesync should be created.
Add external owa legacy URL to the public certificate and install it on both Exchange 2007 and
Exchange 2013 only then owa redirection will work.
You need to Include internal Legacy. Domain.com on Exchange 2007 Certificate for OWA co-
Existence.
Following change needs to be done in Firewall
External OWA URL should be directed to exchange 2013 Internet Facing CAS.
External EWS URL should be directed to  exchange 2013 Internet Facing CAS.
External Autodiscover URL should should be directed to  Exchange 2013 CAS.
External ActivesyncVirtualDirectory should be directed to Exchange 2013 CAS.
External UMvirtualDirectory should be directed to  Exchange 2013 CAS.
Create new NAT rule on firewall for Legacy.domain.com to Exchange 2007 CAS. You can do this as well.By doing this users will be able to log on directly using the URL https://legacy.domain.com/owa with
a mailbox on Exchange 2007.
External and Internal DNS settings
Public DNS - Map all of your external public DNS records (ews,owa,activesync etc.,) to your
exchange 2013 public IP if you have dedicated one for 2013 or FQDN of your internet facing CAS server.
Example:
Current external owa URL (contoso.domain.com) – point it to dedicated exchange 2013 public ip or internet facing exchange 2013 CAS FQDN.
Current External Autodiscover – point it to dedicated exchange 2013 public ip or internet
facing exchange 2013 CAS FQDN
Internal DNS – Configure the Exchange 2007 to point SCP AutoDiscoverURI to Exchange 2013 Client
Access FQDN by changing DNS entry for Autodiscover.domain.com to exchange 2013 CAS sever Ip
address
The internal DNS records should point to the internal host name and IP address of your Exchange
2013 Client Access server
Make sure that legacy.contoso.com resolves to CAS2007 in internal and external DNS.
Authentication Settings:
This part is little bit tricky. You need to plan according to your organization. If you have FBA configured in TMG or ISA server then you need to configure accordingly.
Set the owa virtual directory authentication only to  Basic in exchange 2007.
In exchange 2013 set owa virtual directory to only (Windows Authentication) or only (form-based authentication) or only (Basic, No redirection, SSL Enabled) depends according to your setup.
Things to check:
If you have redirection configured in IIS on the Exchange 2007 Server Make sure that the above
Virtual Directories doesn’t have it configured.
If you have FBA enabled on ISA or TMG then disable FBA on Exchange 2013 CAS else users will be prompted twice for authentication
For further references you can refer my article below
http://exchangequery.com/2014/09/24/owaews-configuration-in-exchange-20132007-coexistence/
Remember to mark as helpful if you find my contribution useful or as an answer if it does answer your question.That will encourage me - and others - to take time out to help you Check out my latest blog posts on http://exchangequery.com Thanks Sathish (MVP)