Split horizon is a method of preventing a routing loop in a network. The basic principle is simple: Information about the routing for a particular packet is never sent back in the direction from which it was received.
Split horizon can be achieved by means of a technique called poison reverse. This is the equivalent of route poisoning all possible reverse paths - that is, informing all routers that the path back to the originating node for a particular packet has an infinite metric. Split horizon with poison reverse is more effective than simple split horizon in networks with multiple routing paths, although it affords no improvement over simple split horizon in networks with only one routing path.