The Exchange 2016 migration for Not Real University is at the stage where they are ready to cut over their client access namespaces to point to Exchange 2016. The cutover itself is just a DNS change for internal namespaces, and a firewall change for external namespaces, but it is a high impact change as it will result in all of your clients connecting to Exchange 2016 for HTTPS services. Any Exchange 2010 mailbox users will continue making RPC connections to the Exchange 2010 CAS array (RPCClientAccessServer). If public folders are still hosted on Exchange 2010 the Outlook clients for Exchange 2013/2016 mailbox users will be proxied to Exchange 2010 for public folder access.
As with any DNS change, rolling the change back is as simple as pointing the DNS record back to its previous value. To ensure a quick change and, if necessary, a quick rollback, you should lower the TTL of your DNS records to less than 5 minutes. A very low value of 1 minute is often recommended. When the change has been proven successful after a few days or weeks, you can increase that TTL value to something higher.
For the external namespace cutover, rolling back the change is usually as simple as reversing the firewall rule changes.
If the external namespace being used, for example mail.notrealuniversity.com, is also the name used for the MX record for inbound mail flow from the internet, then any DNS or firewall changes will also impact inbound mail flow. That is unless your firewall is able to NAT the SMTP port (TCP 25) separately to the HTTPS port (TCP 443), which most firewalls can.
Before the change is made, Not Real University has the following client access flow:
- Inbound HTTPS connections are NATed to the Exchange 2013 server. Internal HTTPS connections also resolve to the Exchange 2013 server. Exchange 2013 proxies HTTPS connections to Exchange 2010 and 2016 (there are no 2016 mailboxes at this time, but 2013 is capable of “up proxying” to 2016).
- SMTP mail flow is unaffected by the changes planned for client access namespaces.
After the DNS and firewall changes, the client access protocol flow for Not Real University is as follows:
- Inbound HTTPS connections are NATed to the Exchange 2016 server. Internal HTTPS connections also resolve to the Exchange 2016 server. Exchange 2016 proxies HTTPS connections to Exchange 2010 and 2013, and public folder connections to Exchange 2010.
- RPC connections for Exchange 2010 mailbox users resolve to the Exchange 2010 server.
As an example, here is the Outlook connection status dialog box for an Exchange 2013 mailbox user after the client access cutover has been performed.
When the client access cutover has been completed and successfully tested, the mail flow cut over can also be performed, which is what we'll look at next.