|  | 11.3 | Distance-Vector Routing
| 11.3.7 | The solution of 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 it sent. 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. As shown in the Figure , 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.