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7.1 | |
Distance Vector Routing | |
| 7.1.6 | |
Avoiding routing loops with triggered updates |
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New routing tables are sent to neighboring
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 that,
in turn, generate triggered updates notifying their adjacent neighbors
of the change. When a route fails, an update is sent immediately
rather than waiting on the update timer to expire. Triggered updates,
used in conjunction with route poisoning, ensure that all routers know
of failed routes before any holddown timers can expire. Triggered updates go ahead and send updates because routing information has changed not waiting for the timer to expire. The router sends another routing update on its other interfaces rather than waiting on the routing update timer to expire. This causes the information about the status of the route that has changed, to be forwarded and starts the holddown timers more rapidly on the neighboring routers. The wave of updates propagates throughout the network. Issuing a triggered update Router C
announces that network 10.4.0.0 is unreachable.
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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.
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