les in this chapter because they are the sim- plest to configure and the easiest to understand. GRE tunnels are solely a means of connecting 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 tun- nel 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 avail- able 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 address assigned to it. A loopback interface on a router is a logical interface within the router that is always up. It can be included in routing protocols and functions like any other interface, with the exception that a loopback interface is always up/up. You can configure multiple loopback interfaces on a router. GRE Tunnels Figure 12-1 shows an example of a simple network in which we will build a GRE tunnel. Four routers are connected. They are all running EIGRP, with all connected networks redistributed into the routing table. The purpose of this example is to show h