split horizon
termnetworking
A distance-vector routing rule: never advertise a route back out the interface you learned it from.
Documented in RFC 1058, the 1988 RIP specification, split horizon breaks the simplest routing loop: without it, two neighbors can advertise a dead route back and forth, counting the metric up toward infinity while traffic ping-pongs. The stricter variant, split horizon with poison reverse, does advertise the route back but with an infinite metric (16 in RIP), which kills the loop faster at the cost of bigger updates. The idea outlived RIP: iBGP applies its own split-horizon rule, refusing to re-advertise a route learned from one iBGP peer to another, which is exactly why iBGP needs a full mesh or route reflectors. Not to be confused with split brain, an unrelated high-availability failure that happens to share the adjective.