Split horizon

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Article

In routing, a split horizon occurs when a route to a destination is advertised to one of the hops used to reach that destination. The problem is that a continual loop occurs if the destination goes down. When this happens the last hop will try to find another way to reach that destination, and ends up with a route that is the same as before.

For example if there are 3 systems called A, B, and C connected in a straight line - B is connected to A and C. To get to C traffic must travel from A to B to finally reach C. If the link between B and C goes down, B will try to find another way to C. We know there is no other possible route, but if B could use A's route to C, it would try that"new" route only to find that route is down, and keep finding it and trying it again.

To prevent this situation, the split horizon rule prevents a route from being advertised back along the route it was learned.

Poisoned split horizon allows advertising back, but it marks the route as unreachable so it will never be used.