it will not work if you are already at the ubuntu, try to stop the autoboot at the start.
It doesn't work at the uboot prompt. That's what I tried when I forgot. There was a warning somehwre on this Wiki that it might not (well, the warning was that it would
not) work, but I tried anyway - and failed. But since I could set it from the OS anyway I didn't really mind - coupled with the fact that it's on a small private network anyway.,..