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Concept
Split horizon route advertisement
Summary
In computer networking, split-horizon route advertisement is a method of preventing routing loops in distance-vector routing protocols by prohibiting a router from advertising a route back onto the interface from which it was learned.
The concept was suggested in 1974 by Torsten Cegrell, and originally implemented in the ARPANET-inspired Swedish network TIDAS.
Here is some basic terminology:
Route poisoning: if a node N learns that its route to a destination D is unreachable, inform that to all nodes in the network by sending them a message stating that the distance from N to D, as perceived by N, is infinite.
Split horizon rule: if a node N uses interface I to transmit to a given destination D, N should not send through I new information about D.
Poison reverse rule: if a node N uses interface I to transmit to a given destination D, N sends through I the information that its cost-to-go to D is infinite.
Whereas under split horizon N does not send any information through I, under poison reverse node N tells a white-lie.
In this example, network node A routes packets to node B in order to reach node C. The links between the nodes are distinct point-to-point links.
According to the split-horizon rule, node A does not advertise its route for C (namely A to B to C) back to B. On the surface, this seems redundant since B will never route via node A because the route costs more than the direct route from B to C. However, if the link between B and C goes down, and B had received a route from A to C, B could end up using that route via A. A would send the packet right back to B, creating a loop. This is the Count to Infinity Problem. With the split-horizon rule in place, this particular loop scenario cannot happen, improving convergence time in complex, highly-redundant environments.
Split-horizon routing with poison reverse is a variant of split-horizon route advertising in which a router actively advertises routes as unreachable over the interface over which they were learned by setting the route metric to infinite (16 for RIP).
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