Explanation:
Split horizon stops routing loops by preventing route update information from being sent back over the interface on which it arrived. Routing loops can occur due to slow convergence and inconsistent routing tables, and can cause excessive use of bandwidth or even complete network failure. Split horizon can prevent routing loops between adjacent routers.

Holddown timers prevent regular update messages from reinstating a route that is unstable. The holddown timer places the route in a suspended, or “possibly down” state in the routing table, and regular update messages regarding this route will be ignored until the timer expires.

Triggered updates are sent as soon as a change in network topology is discovered, as opposed to waiting until the next regular update interval (every 30 seconds in RIP networks). This speeds convergence and helps prevent problems caused by outdated information.

Route poisoning “poisons” a failed route by increasing its cost to infinity (16 hops, if using RIP). Route poisoning is combined with triggered updates to ensure fast convergence in the event of a network change.