No lights on the ports m6220

Hi

I have a Dell m1000e chasis and a1/a2 has poweconnect m6220s. The internal ports 1-16 works and active. But when I plugged the cable on port 17-20 there is not light and show interfaces status says Link down or inactive. I verified the cable is working. Any ideas?

"show interfaces status" says:

--

Gi1/0/17                              N/A     Unknown  Auto  Down   Inactive

---

"sh run" says

interface Gi1/0/17

switchport mode trunk

exit

Thanks

Paras.