[U-Boot] initialisations do by tftp

Hi,

I have a question about initialisations that the tftp command do in u-boot. I try to make a Linux for a Cogent SBC637 and I am able to boot my kernel (save in flash at address 0x10040000) with a NFS if I use the tftp or tftpboot command to load any file in RAM before the "bootm 0x10040000" command but if I didn't do that I have a kernel panic because eth0 is not initialise. Could you tell me what tftp command do exactly? Or how can I resolve this problem.

Thanks in advance for your help.

G.