splithorizon

Split horizon is a routing principle used in computer networks to reduce the likelihood of routing loops in distance-vector routing protocols. The basic idea is that a router should not advertise a route back out of the same interface from which the route information was learned. By preventing information from re-entering the same network segment, the protocol can avoid generating inconsistent routes for neighboring routers on that segment.

Variants of split horizon exist. The standard form simply suppresses advertisements of learned routes on the originating interface. A related variant, split horizon with poison reverse, advertises the route back to the neighbor with an infinite metric, indicating that the path is unusable through that interface. This can help certain network topologies converge more quickly and clearly signal unreachable routes to neighbors.

Scope and applicability. Split horizon is most associated with distance-vector routing protocols over multi-access networks, such as Ethernet segments where several routers can hear each other’s advertisements. On point-to-point links, the risk of routing loops is reduced and many implementations either disable split horizon or apply it differently. In modern networks, link-state and path-vector protocols use other loop-prevention mechanisms and may not rely on split horizon in the same way as traditional distance-vector protocols.

Implementation notes. Split horizon is an operational rule configured within routing software or devices and can typically be enabled, disabled, or tuned per interface or per routing process. It is not a separate protocol, but a procedural safeguard embedded in routing algorithms to improve stability on certain network topologies.

See also: routing loops, poison reverse, distance-vector routing, link-state routing.