EIGRP (Enhanced Interior Gateway Routing Protocol), is a Cisco proprietary routing protocol (until recently). When a router runs EIGRP, it keeps a copy of its neighbors routing table. If I router cannot find a route it its, or its neighbor's routing table, it will query its neighbors who in turn query their neighbors. Exactly how routing protocols work is serious business, but dont worry we are not going to go into that here. Below is the process that I used to setup EIGRP on a Cisco 2811.