Split Horizon: The split horizon is a  method employed by distance vector protocols to prevent network routing LOOPS. The underlying premise is straightforward: NEVER send routing information back in the same direction it came from. It is necessary to have a split-horizon because distance vector protocols like Routing Information Protocol (RIP) are prone to routing loops, which occur when a data packet is caught in an unending loop and routed through the same routers over and over again. Split horizon is frequently used in protocols to avoid loops. Different strategies are used to prevent packet looping in other protocols, such as Open Shortest Path First. 

When split horizon is enabled, a router is prevented from advertising a route back to the router from whence it learnt it. To put it another way, if a router receives routing information from another router, the first router will not broadcast it back to the second router, preventing routing loops.

Example: An example of three routers used to forward packets between networks is shown in the diagram below. The R3 router transmits routing information about the 10.0.0.0/16 network to the R2 router in this simple architecture. This information is received by the R2 router, which modifies its routing table and broadcasts it to the R1 router. The R1 router modifies its routing database when it receives this information.

The modified routing information allows the R1 router to send packets to the 10.0.0.0/16 network VIA the R2 and R3 routers. The R1 router will not be able to advertise this network route back to the R2 router if a split horizon is enabled. If the R1 router does not have split-horizon enabled, it will broadcast the route to the R2 router, which will update its routing table to reflect the network route available through the R1 router.

The presence of the R1 route in the R2 routing database is not an issue in TYPICAL operations because it is plainly a lot more expensive route than a direct R2-to-R3 connection. If the R2-to-R3 connection fails and the R2 router receives a packet from R1 destined for the 10.0.0.0/16 network, the R2 router will return the packet to R1 because the router advertised a functional network path. However, based on its own routing information, the R1 router will just return the packet to the R2 router, resulting in a routing loop that will continue until the packet dies. The R1 router will not advertise the network route to the R2 router if the split horizon is enabled, preventing the routing loop.