re classless. EIGRP and OSPF are classless routing protocols. Poison reverse If a router needs to tell another router that a network is no longer viable, one of the methods it might employ is route poisoning. Consider RIPv1 as an example. Recall that a metric of 16 is considered unreachable. A router can send an update regarding a network with a metric of 16, thereby poisoning the entry in the routing table of the receiving router. When a router 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 protocol EIGRP) use route poisoning, while link-state protocols such as OSPF do not. 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 Chapter 10: Routing Protocols is learned on one permanent virtual circuit (PVC) in a Frame Relay interface, chan- ces 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 pre- ferred 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 net- work is stable. Anytime a link's status changes, the routing protocols must prop- agate 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. As mentioned earlier, many routing protocols exist, 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 an