Thanks a lot! I am sorry for confusing you. 
<br>But the fact is that the 2 boards are connected as the following drawing. The hardware is designed as this. I need to download kernel for board 2 from hard drive of board 1. Usually switch cannot talk to switch without bridge or router. So in this case I think I need VPN in order to download the kernel using tftp.
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<br>&nbsp;HD----CPU1 ----switch1 ------switch 2----CPU2 (uboot)
<br>|----------board 1------ &nbsp; ---|-------------board 2--------------|
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<a href="http://www.nabble.com/U-boot-suppoorts-VPN--t639481.html#a1701279">Re: U-boot suppoorts VPN?</a>