At which layer of the OSI model does ExpressRoute operate?

ExpressRoute operates at Layer 3 (the Network Layer) of the OSI model. This is because ExpressRoute establishes a direct, private connection between your on-premises network and Microsoft's global network, enabling routing of IP packets between these environments. It uses Border Gateway Protocol (BGP) for routing, which is a Layer 3 protocol. While Layer 2 (Data Link Layer) technologies are involved in the underlying physical connection, ExpressRoute's core functionality of IP routing and network peering occurs at Layer 3. Layer 5 (Session Layer) and Layer 7 (Application Layer) are too high in the OSI model, dealing with session management and application-specific protocols, respectively, which are beyond ExpressRoute's scope.