DHCP renew before half the lease time

[Staffan.Ungsgard at teliasonera.com]

I have a device which ignores the lease time and sends out requests to get its address renewed long before half the lease time has expired.
The dhcp server then NAKs the request. One second before half the lease time has expired the client makes a broacast request for the address, and that request is honored.
Is this the correct behaviour - should the dhcp server NAK requests coming to soon, or should I look for a solution on the problem elsewhere.
Best Regards

-- Staffan