Final answer:
The hold-down timer is a loop-prevention mechanism that overrides other mechanisms by holding off any updates for a route for a certain period, avoiding the formation of routing loops.
Step-by-step explanation:
The loop-prevention mechanism that serves as an override of another loop-prevention mechanism is the hold-down timer. This mechanism is used in networking to prevent routing loops during the convergence process after a topology change. In routing protocols like RIP (Routing Information Protocol), a router will hold any updates that might affect routes for a certain duration of the hold-down timer, to avoid the possibility of a routing update that reintroduces a loop.
When a route becomes unreachable, the router marks the route as invalid but does not remove it immediately. It starts the hold-down timer, during which it ignores any possibly bad updates that might arrive for that route. The hold-down timer ensures that no premature decision is taken regarding the state of the route, thus preventing the formation of routing loops.
Poison reverse and split horizon are other loop-prevention mechanisms. Split horizon prevents information about routes from being advertised back onto the interface from which it was learned, whereas poison reverse is a complement to split horizon in which a route is advertised back on the interface it was learned from with a metric that indicates an infinite distance, effectively discouraging its use.