Ignoring an update with a
higher metric when a holddown is in effect enables more time for the knowledge of the change
to propagate through the entire network.
4. During the hold-down period, routes appear in the routing table as ???possibly down.???
Figure 3-18 illustrates the hold-down timer process.
Figure 3-18 Hold-Down Timers
Route Maintenance Using Triggered Updates
In the previous examples, routing loops were caused by erroneous information calculated as a
result of inconsistent updates, slow convergence, and timing. If routers wait for their regularly
scheduled updates before notifying neighboring routers of network catastrophes, serious problems
can occur, such as loops or traf?¬?c being dropped.
Normally, new routing tables are sent to neighboring routers on a regular basis. A triggered update
is a new routing table that is sent immediately, in response to a change. The detecting router
immediately sends an update message to adjacent routers, which, in turn, generate triggered
updates notifying their adjacent neighbors of the change. This wave propagates throughout the
portion of the network that was using the affected link. Figure 3-19 illustrates what takes place
when using triggered updates.
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