Split horizon
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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.
In the example on the right, A uses the path via B to reach C.
A-B-C.png
A cannot advertise its route for C 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; A would send the packet right back to B, creating a loop. With split horizon, this particular loop scenario cannot happen.
An additional variation of split horizon does advertise the route back to the router that is used to reach the destination, but marks the advertisement as unreachable. This is called split horizon with poison reverse.
Split horizon is commonly used in distance-vector routing protocols, since these don't build a map of the network, and thus have no way to avoid routing loops.
Protocols using split horizon
- RIP
- IGRP
- VPLS uses the split horizon technique to avoid loops in the forwarding plane.de:Split_Horizon