What is split horizon, and how does it prevent routing loops in DV protocols?

• A. Advertise every learned route onto every interface.
• B. Limit updates to a single interface.
• C. Do not advertise a learned route back onto the interface from which it was learned.
• D. Do not advertise a learned route back onto the same interface from which it was learned.

Answer: (D)

Split horizon is a rule in distance-vector routing that prevents routing loops by not advertising a learned route back out the same interface from which it was learned. When a router learns a route through a neighbor on a particular link, it withholds that route from updates sent on that same link. This stops the route information from bouncing back to the neighbor and cycling between routers, which could create looping paths. In some setups, a variant called poison reverse is used, where the route is advertised back with an infinite metric to speed convergence, but the basic idea remains: don’t re-advertise a route on the interface it came from.