| 7.1 |  | Distance Vector Routing |
| 7.1.3 |  | Defining a maximum count |

| The invalid updates of Network 1 will continue to loop until some other process stops the looping. This condition, called count to infinity, loops packets continuously around the network in spite of the fundamental fact that the destination network, Network 1, is down. While the routers are counting to infinity, the invalid information allows a routing loop to exist. Without countermeasures to stop the count to infinity process, the distance vector metric of hop count increments each time the packet passes through another router. These packets loop through the network because of wrong information in the routing tables. Distance vector routing algorithms are self-correcting, but a routing loop problem can require a count to infinity. To avoid this prolonged problem, distance vector protocols define infinity as a specific maximum number. This number refers to a routing metric which may simply be the hop count. With this approach, the routing protocol permits the routing loop to continue until the metric exceeds its maximum allowed value. The graphic shows the metric value as 16 hops. This exceeds the distance vector default maximum of 15 hops so the packet is discarded by the router. In any case, when the metric value exceeds the maximum value, Network 1 is considered unreachable.
| Web Links Avoiding Counting To Infinity In Distance Vector Routing http://www.uni-koblenz.de/~steigner/ labor/papers/ ripmti.pdf