Split horizon route advertisement

The concept was suggested in 1974 by Torsten Cegrell, and originally implemented in the ARPANET-inspired Swedish network TIDAS.

[1][2][3] Here is some basic terminology: Whereas under split horizon N does not send any information through I, under poison reverse node N tells a white-lie.

With the split-horizon rule in place, this particular loop scenario cannot happen, improving convergence time in complex, highly-redundant environments.

The effect of such an announcement is to immediately remove most looping routes before they can propagate through the network.

The basic idea of poison reverse is to make sure that a path does not turn back into the same node if a cost has changed within the network.

As long as Z routes via node Y to get to X, Z will tell a white lie to Y: Z will announce to Y an infinite cost to the destination X.

The poison reverse kicks in when a node broadcasts its distance vector to its neighbors.

Poison reverse solves the count-to-infinity problem since if the link between Y and X has its cost increased to, say, 70, then Y and Z will not bounce between each other and instead directly try another path.

Alternatively, if poison reverse is not used, when the link between Y and X has its cost increased, Y will announce the news to Z.