To connect your Azure virtual network and your on-premises network via ExpressRoute, you must create a virtual network gateway first. A virtual network gateway serves two purposes: exchange IP routes between the networks and route network traffic.

When you create a virtual network gateway, you need to specify the gateway version (SKU) that you want to use. When you select a higher gateway version, more CPUs and network bandwidth are allocated to the gateway, and as a result, the gateway can support higher network throughput to the virtual network.

ExpressRoute virtual network gateways can use the following versions:

You can only upgrade from Standard to HighPerformance gateway without downtime. If you want to downgrade or upgrade to UltraPerformance Gateway you have to re-create it which results in a downtime.