Create a tunnel on both routers
(config)#interface tunnel 0
(config-if)#ip address x.x.x.x x.x.x.x
(config-if)#tunnel mode gre ip ( that is what enable the tunnel and is the default )
(config-if)#tunnel source ( the source IP address of the interface that is directly connected )
(config-if)#tunnel destination ( destination IP address of the interface that is directly connected )
Then create a static route or use any routing protocols .
The Mode command is what Enable the tunnel !
When having multiple Tunnel for a same destination you can use the following command to choose the path in case of Link Failure:
(config-if)#backup delay {enable-delay-period | never} {disable-delay-period | never}
(config-if)#backup interface “interface you want to become primary if interface goes down”
