Your Office 365 Outlook connection will do a DNS lookup and Microsoft will use the GEO location of that lookup to connect you to your ‘nearest’ Microsoft Data-center. Outlook will connect to an Exchange CAS server based on the DNS query and use Microsoft’s fast Data-center to data-center backbone network to connect you to the data-center where your Exchange mailbox data is located. Generally, this works well, however, you may not always under all circumstances be connected to the closest data-center.
Using the picture below, consider a case in an enterprise where users that will use Office365 are located in Dallas TX (BLUE), and an Office365 Tenant has been setup for them in the San Antonio data center (PURPLE).
However, all traffic external to the organization goes from Dallas to Phoenix (RED), on a private connection, before reaching the Internet.
Users in Dallas, when doing DNS lookups for hosts on the Internet end up using DNS servers in Phoenix (because that is where data leaves the company network).
Users in Dallas will receive IP addresses of the nearest Office365 data center as San Jose (ORANGE). From there, the data travels all the way back to San Antonio (PURPLE) where the Tenant is hosted.
If the internal DNS servers are not adjusted to point “outlook.office365.com” to the San Antonio data center (PURPLE), users will have a horrible experience due to the many network hops and latency in between.
Changing the DNS servers will cause Dallas users to jump directly to the truly “nearest” Office365 data center.
Credits go to PRIASOFT for all of the above information.
If you update your local DNS to force traffic to a particular data center, make sure to periodically check to see if Microsoft updates their DNS. Every organization will need to weigh the benefits of the speed increase with the overhead of having to maintain local DNS records from outlook.office365.com.
Free Download of tool to determine your closest MSFT Data Center based on TCP ping, courtesy of PRIASOFT.
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