| 7.1 |  | Distance Vector Routing |
| 7.1.4 |  | Elimination routing loops through split horizon |

| Another possible source for a routing loop occurs when incorrect information that has been sent back to a router contradicts the correct information that the router originally distributed. Here is how this problem occurs: Router A passes an update to Router B and Router D, indicating that Network 1 is down. Router C, however, transmits an update to Router B, indicating that Network 1 is available at a distance of 4, by way of Router D. This does not violate split-horizon rules. 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 advising Router A of the new route to Network 1. 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; and 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. Split-horizon attempts 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 thus reduces incorrect routing information and reduces routing overhead.
| Web Links Configuring RIP http://www.cisco.com/en/US/products/sw/ iosswrel/ps1831/products_configuration_guide_ chapter09186a00800d97f7.html#xtocid20