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Summary routes have another important benefit. The router will keep its summary route as long as any of its subnets exist. This means that the summary route is as stable as the most stable route in the summarized range. Without summarization, if there is one route that repeatedly flaps up and down, the routing protocol must propagate every transition throughout the network. But a summary route can hide this instability from the rest of the network. The routing protocol doesn’t need to waste resources installing and removing the flapping route, which improves overall network stability.


Cover of Cisco IOS Cookbook, 2nd Edition


Benefits of summary routes