Split horizon is best explained with an example. You have three routers linked serially, let’s call them A, B, and C. A is only linked to B, C is only linked to B, and B is linked to both A and C. To get to C, A must go through B. If the link to C goes down, it is possible that B will try to use A’s route to get to C. This route is A-B-C, so it will not work. However, if B tries to use it this begins an endless loop.

This situation is called a split horizon because from B’s point of view the horizon stretches out in each direction, but in reality it only is on one side.

Poison reverse is the method used to prevent routes from running into split horizon problems. Poison reverse “poisons” routes away from the destination that use the current router in their route to the destination. This “poisoned” route is marked as unreachable for routers that cannot use it. In IS-IS this means that route is marked with a distance of 16.