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There actually is a reason behind the “wait 30 seconds” thing, and that reason is capacitors. Capacitors hold an electric charge, and it takes a bit for them to discharge. Ever notice LEDs on your computer staying on for a few seconds after you power it down? That’s the residual charge from capacitors. The other reason tech support says 30 seconds is that they actually want you to wait about 10 seconds for the capacitors to discharge, but if they say 10 seconds, many people will only wait for 5. It might also be to give the CMTS enough time to purge the association with your modem and start afresh.