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This article describes the control plane analysis of the test setup. You can also review the test setup configuration and the data plane analysis of the test setup.
Control plane analysis essentially examines routes that are exchanged between networks within a topology. Control plane analysis can help you understand how different networks view the topology.
Hub and spoke virtual network perspective
The following figure illustrates the network from the perspective of a hub virtual network and a spoke virtual network (highlighted in blue). The figure also shows the autonomous system number (ASN) of different networks and routes that are exchanged between different networks:
The ASN of the virtual network's Azure ExpressRoute gateway is different from the ASN of Microsoft Enterprise edge routers (MSEEs). An ExpressRoute gateway uses a private ASN (a value of 65515) and MSEEs use public ASN (a value of 12076) globally. When you configure ExpressRoute peering, because MSEE is the peer, you use 12076 as the peer ASN. On the Azure side, MSEE establishes eBGP peering with the ExpressRoute gateway. The dual eBGP peering that the MSEE establishes for each ExpressRoute peering is transparent at the control plane level. Therefore, when you view an ExpressRoute route table, you see the virtual network's ExpressRoute gateway ASN for the VNet's prefixes.
The following figure shows a sample ExpressRoute route table:
Within Azure, the ASN is significant only from a peering perspective. By default, the ASN of both the ExpressRoute gateway and the VPN gateway in Azure VPN Gateway is 65515.
On-premises Location 1 and the remote virtual network perspective via ExpressRoute 1
Both on-premises Location 1 and the remote virtual network are connected to the hub virtual network via ExpressRoute 1. They share the same perspective of the topology, as shown in the following diagram:
On-premises Location 1 and the branch virtual network perspective via a site-to-site VPN
Both on-premises Location 1 and the branch virtual network are connected to a hub virtual network's VPN gateway via a site-to-site VPN connection. They share the same perspective of the topology, as shown in the following diagram:
On-premises Location 2 perspective
On-premises Location 2 is connected to a hub virtual network via private peering of ExpressRoute 2:
Next steps
Review the test setup for the topology, the Azure networking components it uses, and the shared guidance about running ExpressRoute and a site-to-site VPN in tandem and extending back-end connectivity to spoke virtual networks and branch locations.
Learn about the data plane analysis of the test setup and Azure network monitoring feature views.
See the ExpressRoute FAQ to:
Learn how many ExpressRoute circuits you can connect to an ExpressRoute gateway.
Learn how many ExpressRoute gateways you can connect to an ExpressRoute circuit.
Learn about other scale limits of ExpressRoute.