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You would use this when you want to ensure that the static route always remains in the routing table, even if the next-hop interface is down. There is sometimes a danger that the dynamic routing protocol will install a route that you do not want to use, and it may be preferable to drop the packets than to use the dynamic route. For example, if you had a private link to another IP network, and this link went down, you might not want your routers to try to find a path via the public Internet, even if one were advertised. This is sort of the opposite of the floating static route of Recipe 5.5.

From

Cover of Cisco IOS Cookbook, 2nd Edition

Note

Use of permanent keywork in static route configs