Gateways are used for routing network traffic between a virtual network and another network, either local or remote. Gateways can be used to:

• Create secure site-to-site IPsec connections between SDN virtual networks and external customer networks over the Internet.
• Create Generic Routing Encapsulation (GRE) connections between SDN virtual networks and external networks. The difference between site-to-site connections and GRE connections is that the latter is not an encrypted connection. For more information about GRE connectivity scenarios, see GRE Tunneling in Windows Server.
• Create Layer 3 connections between SDN virtual networks and external networks. In this case, the SDN gateway simply acts as a router between your virtual network and the external network.

Gateways use Border Gateway Protocol to advertise GRE endpoints and establish point-to-point connections. SDN deployment creates a default gateway pool that supports all connection types. Within this pool, you can specify how many gateways are reserved on standby in case an active gateway fails.

For more information, see Software Defined Networking (SDN) in Azure Stack HCI and Windows Server.