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Route flapping occurs when a router's
interface alternates rapidly between the "up" and "down" states. This can be caused by a number of factors,
including a faulty interface or poorly terminated media.
Summarization can effectively insulate upstream routers from route
flapping problems. Consider RTC in the figure. If RTC's interface connected to the
200.199.56.0 network goes down, RTC will remove that route from its table. If the routers
were not configured to summarize, RTC would then send a triggered update to RTZ about
the removal of the specific network, 200.199.56.0. In turn, RTZ would update the next
router upstream, and so on. Every time these routers are updated with new information,
their processors must go to work. It is possible (especially in the case of OSPF routing)
that the processors can work hard enough to noticeably impact performance. Now, consider the
impact on performance if RTC's interface to network 200.199.56.0 comes back up
after only a few seconds. The routers update each other and recalculate.
In addition, what
happens when RTC's link goes back down seconds later? And then back up? This is route
flapping, and it can cripple a router with excessive updates and recalculations.
However, the summarization configuration prevents RTC's route
flapping from affecting any other routers. RTC updates RTZ about a supernet (200.199.56.0
/21) that includes eight networks (200.199.56.0 through 200.199.63.0). The loss of one
network does not invalidate the route to the supernet. While RTC may be kept busy
dealing with its own route flap, RTZ (and all upstream routers) do not notice a thing.
Summarization effectively insulates the other routers from the problem of route
flapping.
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