DHCP problem in Ubuntu 10

Hi All,

I have a BeagleBoard xM, I can install Ubuntu 10 and boot it according to
http://elinux.org/BeagleBoardUbuntu

The ubuntu detail is:
[ 0.000000] Linux version 2.6.37-x2 (root@beagle-256mb-0) (gcc version 4.4.5 (Ubuntu/Linaro 4.4.4-14ubuntu5) ) #1 SMP PREEMPT Wed Feb 2 20:59:40 UTC 2011

After login, I use this command: ifconfig -a :
usb0 Link encap:Ethernet HWaddr ea:69:5f:de:13:c4
UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1
RX packets:0 errors:0 dropped:0 overruns:0 frame:0
TX packets:0 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:1000
RX bytes:0 (0.0 B) TX bytes:0 (0.0 B)

usb1 Link encap:Ethernet HWaddr d6:8a:3f:e0:93:5c
BROADCAST MULTICAST MTU:1492 Metric:1
RX packets:0 errors:0 dropped:0 overruns:0 frame:0
TX packets:0 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:1000
RX bytes:0 (0.0 B) TX bytes:0 (0.0 B)

But when run DHCP command:
ubuntu@omap:~$ sudo dhclient usb0
sudo: timestamp too far in the future: Jan 1 00:00:00 1985
[sudo] password for ubuntu:

Listening on LPF/usb0/ea:69:5f:de:13:c4
Sending on LPF/usb0/ea:69:5f:de:13:c4
Sending on Socket/fallback
DHCPDISCOVER on usb0 to 255.255.255.255 port 67 interval 3

DHCPDISCOVER on usb0 to 255.255.255.255 port 67 interval 13
No DHCPOFFERS received.
No working leases in persistent database - sleeping.

DHCP can success when using usb1:
ubuntu@omap:~$ sudo dhclient usb1

Listening on LPF/usb1/d6:8a:3f:e0:93:5c
Sending on LPF/usb1/d6:8a:3f:e0:93:5c
Sending on Socket/fallback
DHCPREQUEST of 172.30.99.255 on usb1 to 255.255.255.255 port 67
DHCPREQUEST of 172.30.99.255 on usb1 to 255.255.255.255 port 67
DHCPNAK from 172.30.0.254
DHCPDISCOVER on usb1 to 255.255.255.255 port 67 interval 3
DHCPOFFER of 172.30.100.253 from 172.30.0.254
DHCPREQUEST of 172.30.100.253 on usb1 to 255.255.255.255 port 67
DHCPACK of 172.30.100.253 from 172.30.0.254
bound to 172.30.100.253 – renewal in 10430 seconds.

Then I can access internet.

Why usb1 can get IP but usb0 can’t?
I raise this problem because I want to use NFS,

In u-boot, I use this command:
setenv bootcmd ‘mmc init; fatload mmc 0:1 0x80300000 uImage; bootm 0x80300000’
setenv bootargs console=ttyO2,115200n8 noinitrd rw ip=dhcp root=/dev/nfs/ubuntu nfsroot=172.16.0.10:/sdb/nfs/ubuntu,nolock rootdelay=20 raid=noautodetect mpurate=800
boot

The kernel can be load but fail to get IP:

[ 4.592864] usb 1-2.1: new high speed USB device using ehci-omap and address 3
[ 4.796508] ADDRCONF(NETDEV_UP): usb0: link is not ready
************************************************^^^^^^^ Note here: usb0 *******************
[ 5.803771] Sending DHCP requests … timed out!
[ 97.093231] IP-Config: Retrying forever (NFS root)…
[ 97.600799] ADDRCONF(NETDEV_UP): usb0: link is not ready
[ 98.608428] Sending DHCP requests … timed out!

Can you give me some tips?

Thanks