In a dynamic routing environment, routers use special routing protocols to communicate. The purpose of these protocols is simple; they enable routers to pass on information about themselves to other routers so that other routers can build routing tables. There are two types of routing protocols used the older distance vector protocols and the newer link state protocols.
Distance Vector Routing
The two most commonly used distance vector routing protocols are both called Routing Information Protocol (RIP). One version is used on networks running TCP/IP. The other, sometimes referred to as IPX RIP, is designed for use on networks running the IPX/SPX protocol.
RIP works on the basis of hop counts. A hop is defined as one step on the journey to the data's destination. Each router that the data has to cross to reach its destination constitutes a hop. The maximum number of hops that RIP can accommodate is 15. That is to say that in a network that uses RIP, all routers must be within 15 hops of each other to communicate. Any hop count that is in excess of 15 is considered unreachable.
Distance vector routing protocols operate by having each router send updates about all the other routers it knows about to the routers directly connected to it. These updates are used by the routers to compile their routing tables. The updates are sent out automatically every 30 or 60 seconds. The actual interval depends on the routing protocol being used. Apart from the periodic updates, routers can also be configured to send a triggered update if a change in the network topology is detected. The process by which routers learn of a change in the network topology is known as convergence.
Although distance vector protocols are capable of maintaining routing tables, they have three problems. The first is that the periodic update system can make the update process very slow. The second problem is that the periodic updates can create large amounts of network trafficmuch of the time unnecessarily as the topology of the network should rarely change. The last, and perhaps more significant, problem is that because the routers only know about the next hop in the journey, incorrect information can be propagated between routers, creating routing loops.
Two strategies are used to combat this last problem. One, split horizon, works by preventing the router from advertising a route back to the other router from which it was learned. The other, poison reverse (also called split horizon with poison reverse), dictates that the route is advertised back on the interface from which it was learned, but that it has a metric of 16. Recall that a metric of 16 is considered an unreachable destination.