unnel in the sense that VPN or GRE would be considered a tunnel, SSH can be used to access remote devices in addition to the one to which you have connected. While this does not have a direct application on Cisco routers, the concept is similar to that of VPN and GRE tunnels, and thus worth mentioning. I use SSH to access my home network instead of a VPN. Tunnels can encrypt data SO that only the other side can see it, as with SSH; or they can make a remote network appear local, as with GRE; or they can do both, as is the case with VPN. GRE tunnels will be used for the examples in this chapter because they are the simplest to configure and the easiest to understand. GRE tunnels are solely a means of connect- ing remote networks as if they were local networks-they enable a remote interface on another router to appear to be directly connected to a local router, even though many other routers and networks may separate them. GRE does not encrypt data. GRE Tunnels To create a GRE tunnel, you must create virtual interfaces on the routers at each end, and the tunnel must begin and terminate at existing routable IP addresses. The tunnel is not a physical link-it is logical. As such, it must rely on routes already in place for its existence. The tunnel will behave like a physical link in that it will need an IP address on each side of the link. These will be the tunnel interface IPs. In addition, as the link is virtual, the tunnel will need to be told where to originate and terminate. The source and destination must be existing IP addresses on the routers at each end of the tunnel. The best way to guarantee that the tunnel's source and destination points are available is to use the loopback interfaces on each end as targets. This way, if there are multiple paths to a router, the source and destination points of the tunnel are not dependent on any single link, but rather on a logical interface on the router itself. Loopback interfaces are different from loopback IP addresses. A loop- back interface can have any valid IP ad