Split Horizon

Split horizon is a technique used to avoid routing loops . With split horizon, when a route is learned on an interface, that route is not advertised back out on the same interface. For ex-ample, in Figure 2-1, Router 1 receives an update about Network X with a metric of 1 from the neighboring Router 2. Router 1 will not advertise Network X back to Router 2 if split horizon is enabled. If split horizon is disabled, however, Router 1 will advertise Network X with a metric of 2 to Router 2. If Network X fails, Router 2 will think that Router 1 has a better way to get to X, so it will send the packet destined to Network X toward Router 1, creating a black hole.

Figure 2-1. An Example of Split Horizon