split horizon is a mechanism to avoid routing loops in ditance-vector routing protocols by reducing incorrect routing information and routing overhead. The concept of it's work is very simple: a route will never advertise by a router to the interface which it was learned from. It is simple to implement and some distance-vector routing protocols are using it. the RIP and VPLS is two important routing protocols (supported by Mikrotik RouterOS until this article writing time), which use split horizon.
--Vahid Alimohamadi 19:32, 4 January 2013 (UTC)