Setting up Exchange 2007 and Exchange 2013 coexistence

We are preparing to configure coexistence for an Exchange 2007 and Exchange 2013 server.

We have a Cisco ASA Firewall, a pair of Cisco Ironports in a DMZ, and NATing at the firewall for OWA and ActiveSync.

This weekend we will be updating the URLs on the Exchange 2007 server for Autodiscover, OWA, ActiveSync, and Exchange Web Services as we begin to transition our organization from 2007 to 2013.  Using this link as a point of reference - http://www.expta.com/2014/01/exchange-2013-client-access-coexistence.html - here is the current understanding of what we need to do:

Autodiscover – internal and external DNS point to the Exchange 2013 CAS.  Exchange 2013 will proxy for mailboxes that are on the Exchange 2007 server.

ActiveSync – set to Exchange 2013 as it will proxy for any mailboxes that are on the Exchange 2007 server

Exchange Web Services – set Internal and External URLs to https://legacy.contoso.com/EWS/Exchange.asmx

OWA – set the External URL to point to legacy.contoso.com

A couple of questions:

1) When setting up External DNS for the legacy.contoso.com – does it need its own NATed IP at the firewall or would that point to the Exchange 2013 server as a CNAME or another A record to the existing external IP?

2) If no additional NAT is required, will the Exchange 2013 server automatically redirect to legacy.contoso.com for OWA upon successful authentication or is their supplemental work that needs to be done within IIS to provision for legacy.contoso.com for appropriate redirection?

3) Are there any other considerations we need to make to ensure proper mail flow between mailboxes that still live on the Exchange 2007 server and the outside before migration relative to the DNS and URL changes referenced above?

Thank you in advance for your help.
Fidelity BankAsked:
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Adam BrownSr Solutions ArchitectCommented:
You will need another NATed IP address for external access to OWA using Legacy.domain.com. Without the additional NATed IP the redirect would fail externally. If you don't care about access to OWA externally, you should be okay with forgoing the additional natting, since Exchange 2013 CAS will Proxy to the 2007 CAS server for other services (Like RPC over HTTPS). As for mail flow, Exchange will handle that during installation. There shouldn't be much you need to do to get it going properly. However, you may need to make sure the 2013 servers are listed on your Send connectors. That may not be done for you.

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