The on-premises environment connects to Azure via an ExpressRoute, and communication between AVS and the on-premises environment needs to be established. The recommended (and simplest) way to achieve this is with ExpressRoute GlobalReach.
The ExpressRoute GlobalReach connection is bundled part of the private cloud cost.
If there are multiple on-premises locations connecting with an ExpressRoute multiple Global reach connections can be established.
This graphic shows both the on-premises and AVS ExpressRoutes connecting to the same ExpressRoute gateway, but that is not a requirement.
When this connection is established AVS will learn all the routes being advertised from on-premises and on-premises will learn all the networks which are part of AVS.
There are automated deployment options in-lieu of doing the manual steps outlined below;