Split Horizon
Split horizon is used to avoid routing loops. With Split horizon, when a route is learned through an interface, that route is not sent back out on the same interface. For example, Router1 gets an update about a network with a metric of 3 from the neighboring Router2.Router1 will not advertise network back to Router2 if the split horizon is enabled. If Split horizon is disabled, Router1 will advertise the network with a metric of 4 to Router2. If Router2 loses reachability to the that network, it thinks that Router1 has a better path to get to that network.Then it sends the IP packets to Router1 and Router1 drops the packets.
