DHCP renewal doesn't work

My device uses AT91SAM7X-EX chip and works as a DHCP client with Nut/OS v4.8.7.  It can get IP address from DHCP server after I power it up. But it isn't automatically issuing a renew based on the renewal period issued by my DHCP server. The renewal time is 2 minutes for testing purpose. I connect one computer, one DHCP server and the SAM7X device to a hub in the same network and run Wireshark  to capture messages. Computer sends DHCP request every minute but SAM7x device never sends any request. 

I saw people mentioned DHCP issues related to stack size before. I checked my settings. The value of NUT_THREAD_DHCPSTACK is 1024 in at91sam7x-ek.conf. MIN_DHCP_WAIT is 4000.

I don't know what else I need to check.

Thanks for any help.

Mingshu
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