split horizon
Picture office gossip: it would be silly to tell a coworker a piece of news that you only just heard from that very same coworker. Split horizon is exactly that rule applied to routers: do not advertise a route back out the same interface you learned it from. The aim is to stop two neighbors from feeding each other stale information about a destination they reach through each other.
Concretely, if router A's best path to destination Z goes through neighbor B, then split horizon says A must not tell B 'I can reach Z' — because A's only route to Z is via B itself, so that claim is useless to B and dangerously circular. A stronger variant, poison reverse, has A actively tell B 'my distance to Z via you is infinity', explicitly poisoning the false hope rather than just staying silent. Both target the simplest count-to-infinity loop: with split horizon, B never hears A advertise the route B itself provided, so the two cannot bounce an unreachable destination back and forth.
It matters as a cheap, standard mitigation built into distance-vector protocols like RIP. It speeds convergence after a link fails and prevents the most common two-router loops without any change to the underlying Bellman-Ford computation.
An honest caveat: split horizon (even with poison reverse) only breaks loops between two routers that reach a destination directly through each other. Loops that wind through three or more routers can still count to infinity. So split horizon is a real improvement, not a cure — it is one reason distance-vector survives in small networks, but not a reason to trust it at large scale.
A's route to Z goes via B. Plain split horizon: A simply omits Z when advertising to B. Poison reverse: A explicitly advertises 'Z = infinity' to B, so B immediately knows not to route Z back through A.
Split horizon stays silent about a route to the neighbor it learned it from; poison reverse loudly advertises infinity instead.
Split horizon and poison reverse only cure two-router loops. Loops spanning three or more routers can still count to infinity, so they are mitigations, not guarantees.