2.2.2. Triggered updates

Split horizon with poisoned reverse will prevent any routing loops that involve only two gateways. However, it is still possible to end up with patterns in which three gateways are engaged in mutual deception. For example, A may believe it has a route through B, B through C, and C through A. Split horizon cannot stop such a loop. This loop will only be resolved when the metric reaches infinity and the network involved is then declared unreachable. Triggered updates are an attempt to speed up this convergence. To get triggered updates, we simply add a rule that whenever a gateway changes the metric for a route, it is required to send update messages almost immediately, even if it is not yet time for one of the regular update message. (The timing details will differ from protocol to protocol. Some distance vector protocols, including RIP, specify a small time delay, in order to avoid having triggered updates generate excessive network traffic.) Note how this combines with the rules for computing new metrics. Suppose a gateway's route to destination N goes through gateway G. If an update arrives from G itself, the receiving gateway is required to believe the new information, whether the new metric is higher or lower than the old one. If the result is a change in metric, then the receiving gateway will send triggered updates to all the hosts and gateways directly connected to it. They in turn may each send updates to their neighbors. The result is a cascade of triggered updates. It is easy to show which gateways and hosts are involved in the cascade. Suppose a gateway G times out a route to destination N. G will send triggered updates to all of its neighbors. However, the only neighbors who will believe the new information are those whose routes for N go through G. The other gateways and hosts will see this as information about a new route that is worse than the one they are already using, and ignore it. The neighbors whose routes go through G will update their metrics and send triggered updates to all of their neighbors. Again, only those neighbors whose routes go through them will pay attention. Thus, the triggered updates will propagate backwards along all paths leading to gateway G, updating the metrics to infinity. This propagation will stop as soon as it reaches a portion of the network whose route to destination N takes some other path.

If the system could be made to sit still while the cascade of triggered updates happens, it would be possible to prove that counting to infinity will never happen. Bad routes would always be removed immediately, and so no routing loops could form.

Unfortunately, things are not so nice. While the triggered updates are being sent, regular updates may be happening at the same time. Gateways that haven't received the triggered update yet will still be sending out information based on the route that no longer exists. It is possible that after the triggered update has gone through a gateway, it might receive a normal update from one of these gateways that hasn't yet gotten the word. This could reestablish an orphaned remnant of the faulty route. If triggered updates happen quickly enough, this is very unlikely. However, counting to infinity is still possible.