In distance vector routing, what is the best description of the split horizon rule used by protocols such as RIP?

Introduction / Context:
Split horizon is a classic loop prevention technique used in distance vector routing protocols such as RIP. Routing loops can cause severe instability and wasted bandwidth in a network. Understanding the definition and effect of split horizon is vital for anyone studying routing protocols and preparing for Cisco certification exams.

Given Data / Assumptions:

• We are working with a distance vector protocol like RIP that sends periodic routing updates.
• A router learns routes from its neighbors and advertises those routes back out some interfaces.
• Split horizon is configured on an interface by default in many implementations.
• The goal of split horizon is to reduce the chance of routing loops.

Concept / Approach:
The split horizon rule states that a route learned on a particular interface is not advertised back out of that same interface. In other words, information about a route should not be sent back in the direction where it was originally learned. This prevents situations where routers would bounce routes back and forth, potentially creating routing loops or counting to infinity scenarios. Split horizon is a simple rule but very effective in common topologies such as hub and spoke or partial mesh networks.

Step-by-Step Solution:
Step 1: Consider a router R1 that learns about network 10.0.0.0 from neighbor R2 on interface Serial 0.Step 2: With split horizon enabled, R1 will not include route 10.0.0.0 in RIP updates sent out on Serial 0 back toward R2.Step 3: R1 may still advertise route 10.0.0.0 out other interfaces to different neighbors, which is desirable.Step 4: By not sending the route back to R2 over the same interface, R1 avoids helping form a loop where R2 might believe that R1 has a shorter path.Step 5: Over time, this simple rule contributes to more stable and predictable distance vector routing behaviour.

Verification / Alternative check:
In lab scenarios, you can enable debugging of RIP updates and watch how routes are advertised with and without split horizon. When split horizon is active, you will see that routes learned on a given interface do not appear in updates sent out that same interface. This confirms the enforcement of the rule and illustrates its loop prevention effect.

Why Other Options Are Wrong:
Option B incorrectly associates split horizon with splitting traffic, which is more related to load balancing rather than loop prevention.Option C suggests that split horizon is about downed links, but link failures are handled by timers and triggered updates, not by split horizon directly.Option D describes a behaviour closer to route poisoning and hold down timers, which are separate loop prevention mechanisms.Option E misrepresents split horizon and instead talks about default route advertisement, which is a different concept.

Common Pitfalls:
Learners often confuse split horizon with route poisoning, triggered updates, or hold down timers. All of these techniques address routing loops and convergence but in different ways. Remember that split horizon is specifically about not advertising a route back out the interface on which it was learned.

Final Answer:
The best description of split horizon is that Information about a route is not sent back in the direction from which the original update was received.