Generic Routing Encapsulation protocol can be used as logical point-to-point links, encapsulating most any traffic over a tunnel as unicast. The unicast feature being useful over IPSEC as IPSEC only works over unicast traffic.

Assuming all destinations are routable – creating a basic GRE Tunnel over the following topology:

Simple Topology–

• 3 routers / 2 subnets
• R1 and R3 Loopback0 interfaces
• We will create 172.16.0.0 /30 GRE Tunnel.