حاول تقرأ الشرح واذا مافهمت كتير اشرحلك اياه بكرة ان شاء الله
split horizon: routing information cannot be sent back in the direction from which it was received.it means that when router Router A learns a route from router Router B, Router A has no need to advertise that same route back to Router B.
Look at the diagram.
Distance vector routing protocols use multicast IP addresses.so consider that routerA learns a route from RouterB and RouterA wants to advertise it to other Routers except RouterB(base on split-horizon rule) if on RouterA multipoint interface is configured the advertisement go out of the multipoint interface and all the other router who listen for the multicast IP address receive the route(router B,C,D and E) but it against split horizon rule because routerA shouldn't advertise the router to RouterB (there is no way to advertise the route just to router C ,D and E with one multipoint interface).
but if we configure point to point subinterfaces with separate IP addresses and Networks on Router A then routerA will able to advertise the route just to router C , D and E by sending the advertisement on the appropriate subinterfaces.so in this way router A obeys the split horizon rule.
when we configure point to point subinterfaces we need to configure one seprate IP network address on each subinterface connected to each spoke device.so more than one IP network address is needed.
In each way we need just one physical interface (called: Access Link) which is connected to Frame-relay Switch
Point-to-point subinterfaces offer no greater security compared to a multipoint interface configuration