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  • Jonathan Black thinks this is interesting:

For example, if one side of the link has hello interval set to 5 seconds with default hold time set to 15 seconds and the other side has hello interval set to 30 seconds, the routers will establish neighbor relationship for 15 seconds, and then it will be down for the next 15 seconds.

From

Cover of Implementing Cisco IP Routing (ROUTE) Foundation Learning Guide: (CCNP ROUTE 300-101)

Note

This is incorrect. The hold timer tells the other router how long it should wait before declaring you dead. The hello timer tells you how long you should wait before sending a hello. Thus to get the flapping relationship you must configured the hello timer to be greater than (Possibly equal to) the hold timer on the same router.