| 7.1 |  | Distance Vector Routing |
| 7.1.5 |  | Route poisoning |

| Route poisoning is used by various distance vector protocols in order to overcome large routing loops and offer explicit information when a subnet or network is not accessible. This is usually accomplished by setting the hop count to one more than the maximum. One way to avoid inconsistent updates is route poisoning. When Network 5 goes down, Router E initiates route poisoning by making a table entry for Network 5 as 16, or unreachable. By this poisoning of the route to Network 5, Router C is not susceptible to incorrect updates about the route to Network 5. When Router C receives a router poisoning from Router E, it sends an update, called a poison reverse, back to Router E. This makes sure all routes on the segment have received the poisoned route information. When route poisoning is used with triggered updates it will speed up convergence time because neighboring routers do not have to wait 30 seconds before advertising the poisoned route. Route poisoning causes a routing protocol to advertise infinite-metric routes for a failed route. Route poisoning does not break split horizon rules. Split horizon with poison reverse is essentially route poisoning, but specifically placed on links that split horizon would not normally allow routing information to flow across. In either case, the result is that failed routes are advertised with infinite metrics.
| Web Links Route Poisoning http://www.firewall.cx/distance_vector.php