Split Horizon

As mentioned earlier, route poisoning does not solve the counting-to-infinity problem. Counting-to-infinity can occur when one router has a valid metric that points to an address that is reachable through an intermediate router while the intermediate router has an infinite-distance route to the same address (see Figure 4.1). If routing table updates are sent by both routers at the same in time, the intermediate router will advertise that the route to the destination address is an infinite-distance route while the other router will advertise that the route has a valid metric. Because the two routers use the same update interval between updates, this process repeats itself with the next routing update, with the difference that the valid metric will be incremented by 1 each time until an infinite metric is reached, hence this phenomenon is called counting to infinity.

FIGURE: Count To Infinity

Split horizon solves the counting-to-infinity problem by preventing a router from sending routing updates out the same interface on which it learnt the route. Thus, in Figure 4.1, the router would have learnt the route to the destination address across the link from the intermediate router. With split horizon, that router cannot then send advertisements about the route to the destination address out across the same link. Therefore the intermediate router will not receive the valid metric from the route to the destination address from the other router ad the count to infinity problem will not occur, solving the count-to-infinity problem on a single link.