Cisco GRE Tunnel

Generic Routing Encapsulation (GRE) - RFC 2784 - developed by Cisco is non-secure, stateless (no flow control), site-to-site VPN tunneling protocol. The Protocol Type field holds identifier for a passenger protocol (encapsulated L3 OSI protocol) - defined in RFC 1700 as EtherTypes.

Configuration

R1(config)# interface Tunnel0                              R2(config)# interface Tunnel0     
R1(config)# tunnel mode gre ip                             R2(config)# tunnel mode gre ip        !tunnel interface is GRE over IP
R1(config)# ip address 10.1.1.1 255.255.255.252            R2(config)# ip address 10.1.1.2 255.255.255.252   
R1(config)# tunnel source 209.165.201.1                    R2(config)# tunnel source 198.133.219.87
R1(config)# tunnel destination 198.133.219.87              R2(config)# tunnel destination 209.165.201.1
R1(config)# router ospf 1                                  R2(config)# router ospf 1     
R1(config-router)# network 10.1.1.1 0.0.0.3 area 0         R2(config-router)# network 10.1.1.1 0.0.0.3 area 0

Troubleshooting

show ip interface brief | include Tunnel
show interface Tunnel 0