Split horizon rule

In routing protocols, the split horizon rule states that a router must not readvertise a route to the router from which a route was first learned. It is a means of preventing loops from forming, and is used with distance vector protocols such as Routing Information Protocol (RIP) and the Interior Gateway Routing Protocol, and the path vector protocol, Border Gateway Protocol.

Distance vector

Poisoned reverse

"If two routers have routes pointing at each other, advertising reverse routes with a metric of 16 will break the loop immediately. If the reverse routes are simply not advertised, the erroneous routes will have to be eliminated by waiting for a timeout. However, poisoned reverse does have a disadvantage: it increases the size of the routing messages."