When you want to connect two Azure virtual networks (VNets) so that traffic between them stays on the Microsoft backbone network—and does not traverse the public internet—the correct configuration is Azure Virtual Network Peering.

Key Concept: Virtual Network Peering

According to Microsoft Azure Administrator Documentation (AZ-104 Study Guide & Azure Network Architecture Guide):

“Virtual network peering enables you to seamlessly connect two Azure virtual networks. The traffic between virtual machines in the peered networks uses the Microsoft backbone infrastructure and never traverses the public Internet, providing low latency, high bandwidth, and secure connectivity.”

Types of Peering:

VNet Peering (Intra-region) — connects VNets in the same Azure region.

Global VNet Peering — connects VNets across different Azure regions but within the same Azure cloud.

Both types use the Microsoft backbone network for communication, ensuring high-performance private connectivity.

Why Other Options Are Incorrect:

A. ExpressRoute:

ExpressRoute provides private connectivity between on-premises networks and Azure, not between Azure VNets. It is used for hybrid connectivity, not VNet-to-VNet communication inside Azure.

B. Private Endpoint:

Private Endpoints provide private access to Azure PaaS services (like Azure Storage, SQL Database, etc.), not for connecting two VNets directly.

D. Route Table:

Route tables (User-Defined Routes) control traffic flow within or between subnets/VNets, but they do not ensure traffic uses the Microsoft backbone. Without peering or a gateway, traffic would route via public IPs and the internet.

Official Microsoft Azure Documentation Extract:

From Microsoft Learn – Virtual Network Peering Overview:

“When virtual networks are peered, traffic between them is routed through the Microsoft backbone infrastructure, much like traffic between virtual machines in the same network. This ensures traffic never goes through the public Internet.”

Conclusion:

To guarantee that all traffic between VNet1 and VNet2 traverses Microsoft’s private backbone network, you must configure VNet peering (either local or global depending on regions).