Split horizon prevents routers from advertising a route through the same interface from which the route was learned. Thus split horizon prevents routing loops, not switching loops. By default, split horizon is enabled on all interfaces except those on which Frame Relay encapsulation or Switched Multimegabit Data Service (SMDS) encapsulation is enabled. To enable split horizon, you should issue the ip splithorizon or ip splithorizon eigrpcommand from interface configuration mode.

To disable split horizon, you should issue the no ip splithorizon or no ip splithorizon eigrp command from interface configuration mode.

Spanning Tree Protocol (STP) prevents switching loops on a network. Switching loops can occur when there is more than one switched path to a destination. The spanning tree algorithm determines the best path through a switched network, and any ports that create redundant paths are blocked. If the best path becomes unavailable, the network topology is recalculated and the port connected to the next best path is unblocked.

Poison reverse prevents routing loops by advertising a route as unreachable to the interface from which the route was received. Split horizon is similar to poison reverse in that both methods prevent routing loops. However, poison reverse advertises a route as unreachable to the source interface, whereas split horizon does not. Route poisoning is similar to poison reverse in that both methods prevent routing loops by advertising a route as unreachable. However, route poisoning sends the advertisements to all interfaces, not just to the source interface.

Holddown timers also prevent routing loops. Holddown timers suppress information regarding a better path to a route for a specified period of time. When a router receives a routing update stating that a route is unreachable, the router waits a specified amount of time before accepting routes advertised by other sources.

VLAN Trunking Protocol (VTP), not split horizon, is used to synchronize VTP and virtual LAN (VLAN) configuration information between switches.