Hi Carsten,

despite the lack of any really useful information (logs?) I'll try anyway.

you wrote:
> I'm trying hard to get NetBSD booting on my DEC3000. I set up the tftp
> and bootp servers properly, but system is not getting up anyway.
> The assignment of the ip addresses works, but if the tftp transfer of
> the kernel or whatever should start, it only occurs an error message.
              ^^^^^^^^^^^
> Can somebody give me advice? Thanks in advance.

You did specify the right bootfile in bootptab(5), did you? Making 
guesses as to what the box should actually load indicates you didn't 
specify any bootfile at all. You should put /usr/mdec/netboot somewhere 
where tftpd(8) can find it and set bf= and rp= (root path) in 
bootptab(5) accordingly.

I can only speak from my experience with my AXPpci33. Observed behaviour 
here is (I use dhcpd):

  1. SRM:	BOOTP request						
  2. dhcpd:	BOOTP reply with boot file from alpha:/usr/mdec/netboot
  3. SRM:	loads boot boot file via TFP
  4. netboot:	DHCP DISCOVER with vendor-class-identifier
                 NetBSD:$(uname -m):libsa
  5. dhcpd:	DHCP OFFER
  6. netboot:	DHCP REQUEST with vendor-class-identifier
                 NetBSD:$(uname -m):libsa
  7. dhcpd:	DHCP ACK with root-path
  8. netboot:	mounts root path via NFS and loads kernel /netbsd
  9. kernel:	DHCP DISCOVER with vendor-class-identifier
                 $(uname -s):$(uname -m):kernel:$(uname -r)
10. dhcpd:	DHCP OFFER
11. kernel:	DHCP REQUEST with vendor-class-identifier
                 $(uname -s):$(uname -m):kernel:$(uname -r)
12. dhcpd:	DHCP ACK with root-path
13. kernel:	mounts root from root path and boots

YMMV.

Also check 
http://www.netbsd.org/Documentation/network/netboot/intro.alpha.html

Nino