Avoiding routing loops with triggered updates

7.1.6 This page will explain how triggered updates can be used to prevent routing loops.
New routing tables are sent to neighbor routers on a regular basis. For example, RIP updates occur every 30 seconds. However a triggered update is sent immediately in response to some change in the routing table. The router that detects a topology change immediately sends an update message to adjacent routers. These routers generate triggered updates to notify their adjacent neighbors of the change. When a route fails, an update is sent immediately. Triggered updates, used in conjunction with route poisoning, ensure that all routers know of failed routes before any holddown timers can expire.
Triggered updates do not wait for update timers to expire. They are sent when routing information has changed. A router will immediately send a routing update on its other interfaces. This forwards the information about the route that has changed and starts the holddown timers sooner on the neighbor routers. The wave of updates propagates throughout the network.
Router C issues a triggered update, which announces that network 10.4.0.0 is unreachable.

Upon receipt of this information, Router B announces through interface S0/1 that network 10.4.0.0 is down. In turn, Router A sends an update out interface Fa0/0.
The next page will explain how holddown timers can be used to prevent routing loops.

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