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Elimination routing loops through split-horizon

Elimination routing loops through split-horizon 
7.1.4
This page will explain how split horizon can be used to avoid routing loops.
Some routing loops occur when incorrect information that is sent back to a router contradicts the correct information that the router originally distributed. An example is as follows:
  1. Router A passes an update to Router B and Router D, which indicates that Network 1 is down. However, Router C transmits an update to Router B, which indicates that Network 1 is available at a distance of 4, by way of Router D. This does not violate split horizon rules.
  2. Router B concludes, incorrectly, that Router C still has a valid path to Network 1, although at a much less favorable metric. Router B sends an update to Router A, which informs Router A of the new route to Network 1.
  3. Router A now determines that it can send to Network 1 by way of Router B. Router B determines that it can send to Network 1 by way of Router C. Router C determines that it can send to Network 1 by way of Router D. Any packet introduced into this environment will loop between routers.
  4. Split horizon is used to avoid this situation. If a routing update about Network 1 arrives from Router A, Router B or Router D cannot send information about Network 1 back to Router A. Split horizon reduces incorrect routing information and routing overhead.
The next page will introduce the concept of route poisoning.

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