What is split horizon, and how does it prevent routing loops in distance-vector protocols?

• A. Advertise every learned route onto every interface.
• B. Advertise learned routes only to the next-hop router.
• C. Limit updates to be sent only to directly connected neighbors.
• D. Do not advertise a learned route back onto the same interface from which it was learned.

Answer: (D)

Split horizon is a rule used in distance-vector routing protocols to prevent routing loops. The idea is that when a router learns about a destination from a neighbor on a specific interface, it should not advertise that same route back out through that same interface to the same neighbor. By not sending the learned route back on the path it came from, the network avoids feedback that can create alternating updates and looping paths.

This is why the best description is: do not advertise a learned route back onto the same interface from which it was learned. For example, if a router learns about a destination via a neighbor on one link, it won’t tell that neighbor about that destination on that same link, which stops the potential loop. Some networks also use a related technique called poison reverse to actively mark the route as unreachable on other interfaces, but the core idea of split horizon is simply not re-advertising the learned route on the originating interface.