Split horizon

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In computer networks, distance-vector routing protocols employ the split horizon rule which prohibits a router from advertising a route back out the interface from which it was learned. Split horizon is one of the methods used to prevent routing loops due to the slow convergence times of distance-vector routing protocols.

In this example A uses the path via B to reach C.

A will not 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.

[edit] Protocols using split horizon

• RIP
• IGRP
• EIGRP
• VPLS uses the split horizon technique to avoid loops in the forwarding plane.

Convergence in RIP Internetworks, from Microsoft TechNet