Why ExpressRoute may be a difficult relative for Office 365
Azure ExpressRoute lets you create private connections between Azure datacenters and infrastructure on your premises or in a colocation environment (eg. Equinix) with an SLA of 99.95%. It actually enables you to extend your Datacenter to Azure without any need to go through Internet. They offer more reliability, faster speeds, and lower latencies than typical Internet connections.
Released in 2014 the ExpressRoute has been a huge success for Microsoft, enabling customers to move to Azure with almost same latency of your Datacenter.
Office 365 was Microsoft's answer to Google apps, where all microsoft's greatest treasures are SaaSsified. Launched in 2011 Office 365 offers Exchange, SharePoint, Lync, Yammer, OneDrive and Office Online.
Initially aimed at small business & enterprises the service has grown and provides services to mainstream consumers also.
With the launch of ExpressRoute there were huge requests for Office 365 support, which was granted in 2015. Consumers of ExpressRoute saw the opportunity and began moving their subscriptions with ExpressRoute. A great family union.
Looks like someone at Microsoft pressed the Drama button, integration to Office 365 via ExpressRoute may take a country road if you are not careful.
According to Latest Microsoft Guidelines
Infrastructure and platform services running in Azure will often benefit by addressing network architecture and performance considerations. We recommend ExpressRoute for Azure in these cases. Software as a Service offerings like Office 365 and Dynamics 365 have been built to be accessed securely and reliably via the Internet. Accordingly, we only recommend ExpressRoute for these applications in specific scenarios.
The main reason is Office 365 is hosted in Microsoft Cloud than Azure. And is built to run via public internet. For effective use of Office 365 many requests are followed through Internet. This can create Asymmetric routing and provide bad experience for the users.
To minimize routing and other network complexities, it is recommend that you only use ExpressRoute for Office 365 for the network traffic flows that are required to go over a dedicated connection due to regulatory requirements or for high quality lync experience.
Currently if you want to use ExpressRoute for Office 365, you would need to clear a network assessment from Microsoft and also sign an agreement about the usage scenarios.
I am hopeful that these issues will be ironed out in the future releases paving a new faster dedicated connection to Office 365.