How do I configure GRE tunnels?

Configuring GRE tunnels on Cisco routers is relatively easy—all it takes is a few simple commands. Here's an example of a simple configuration:

Router A:

interface Ethernet0/1

ip address 10.2.2.1 255.255.255.0

interface Serial0/0

ip address 192.168.4.1 255.255.255.0

interface Tunnel0

ip address 1.1.1.2 255.255.255.0

tunnel source Serial0/0

tunnel destination 192.168.4.2

Router B:

interface FastEthernet0/1

ip address 10.1.1.1 255.255.255.0

interface Serial0/0

ip address 192.168.4.2 255.255.255.0

interface Tunnel0

ip address 1.1.1.1 255.255.255.0

tunnel source Serial0/0

tunnel destination 192.168.4.1