r receives a poison update, it returns the same update to the sending router. This reflected route poisoning is called poison reverse. Distance-vector routing protocols (including the hybrid pro- tocol EIGRP) use route poisoning, while link-state protocols such as OSPF do not. Specific Routing Protocols Split horizon Split horizon is a technique used by many routing protocols to prevent routing loops. When split horizon is enabled, routes that the routing protocol learns are not advertised out the same interfaces from which they were learned. This rule can be problematic in virtual circuit topologies, such as frame relay or ATM. If a route is learned on one permanent virtual circuit (PVC) in a frame-relay interface, chances are the other PVC needs the update, but will never receive it because both PVCs exist on the same physical interface. Frame-relay subinterfaces are often the preferred method of dealing with split horizon issues. Convergence A network is said to be converged when all of the routers in the network have received and processed all updates. Essentially, this condition exists when a network is stable. Any time a link's status changes, the routing protocols must propagate that change, whether through timed updates, or triggered updates. With timed updates, if updates are sent, but no changes need to be made, the network has converged. There are many routing protocols in existence, but luckily, only a few are in wide- spread use. Each has its own idiosyncrasies. In the following sections, I'll cover the basic ideas behind the more common protocols, and show how to configure them for the most commonly seen scenarios. There is no right or wrong way to configure rout- ing protocols, though some ways are certainly better than others. When designing any network, remember that simplicity is a worthy goal that will save you countless hours of troubleshooting misery. The Routing Information Protocol (RIP) is the simplest of the routing protocols in use today. While I like to tell my clients that simple is go