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The split horizon rule prohibits a router from advertising a route through an interface that the router itself is using to reach the destination.
This is used to help prevent routing loops. For example, "A" reaches "C" via "B". "A" cannot advertise its route back to "B". On the surface, this seems redundant since "B" will never use "A"'s route because it costs more than "B"'s route to "C". However, if "B"'s route to "C" goes down, "B" could end up using "A"'s route, which goes through "B". This would then create a loop. With split horizon, this particular loop scenario would never happen.
See also: Routing information protocol
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