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| 2.4 |  | Route Summarization |
| 2.4.2 |  | Route flapping |

Route flapping occurs when a router 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 the RTC 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 Open Shortest Path First (OSPF) routing, that the processors can work hard enough to noticeably impact performance. Now, consider the impact on performance if the RTC 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 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 the RTC 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, are unaware of any downstream problem. Summarization effectively insulates the other routers from the problem of route flapping.