This is how to configure ip gre tunnel from linux foundation: http://www.linuxfoundation.org/collab...
This is how to configure ip gre tunnel in cisco: https://supportforums.cisco.com/docum...
This is how to add a loopback interfaceand add a route in linux : http://askubuntu.com/questions/444124...
Tunneling can be used to focus traffic to a certain destination.