| 7.1 |  | Distance Vector Routing |
| 7.1.7 |  | Preventing routing loops with holddown timers |

| A count to infinity problem can be avoided by using holddown timers: When a router receives an update from a neighbor indicating that a previously accessible network is now inaccessible, the router marks the route as inaccessible and starts a holddown timer. If at any time before the holddown timer expires an update is received from the same neighbor indicating that the network is again accessible, the router marks the network as accessible and removes the holddown timer. If an update arrives from a different neighboring router with a better metric than originally recorded for the network, the router marks the network as accessible and removes the holddown timer. If at any time before the holddown timer expires an update is received from a different neighboring router with a poorer metric, the update is ignored. Ignoring an update with a poorer metric when a holddown timer is in effect allows more time for the knowledge of a disruptive change to propagate through the entire network.
| Web Links Holddown http://www.cs.berkeley.edu/~kfall/ EE122/lec16/ sld035.htm