Routing Fundamentals — Split Horizon (Distance-Vector Loop Prevention) In distance-vector routing (for example, RIP), what is the precise meaning of the split horizon rule?

Introduction / Context:
Split horizon is a classic loop-prevention technique used by distance-vector routing protocols such as RIP and IGRP. Understanding the exact definition helps you predict which routes are advertised on which interfaces and why certain updates are intentionally suppressed.

Given Data / Assumptions:

• The protocol is distance-vector (for example, RIP).
• Routers periodically send updates out interfaces.
• We want to prevent routing information from boomeranging back to its source and creating loops.

Concept / Approach:
Split horizon states: do not advertise a route back out the interface on which it was learned. This suppresses misleading advertisements that could cause a neighbor to believe you have a better path to a network when your information originally came from that same neighbor. Some implementations extend this with poison reverse, which advertises the route back with an infinite metric to make the suppression explicit.

Step-by-Step Solution:
Identify the interface on which a route was learned (incoming interface).When generating periodic updates, omit that route on the same outgoing interface (split horizon).Optionally, use poison reverse to send the route back with metric 16 (in RIP) instead of omitting it.Result: neighbors do not re-install a route that would create a loop.

Verification / Alternative check:
Enable protocol debugging (for example, debug ip rip) on a lab router and observe that routes learned on Serial0 are not re-advertised out Serial0 unless poison reverse is configured.

Why Other Options Are Wrong:

• B: Refers to a physical topology myth; split horizon is not about bus networks.
• C: Describes update suppression to a downed link, not the learned-on-interface rule.
• D: Sounds like holddown/poisoning behavior, not the definition of split horizon.
• E: Summarization is separate (for example, auto-summary in older RIP/IGRP) and unrelated to split horizon.

Common Pitfalls:
Confusing split horizon with holddown timers or route poisoning; in practice, networks often use a combination of these mechanisms, but their definitions differ.

Final Answer:
Information about a route should not be sent back in the direction from which the original update came.