I have more details on my attempt to boot NEC riscserver 2200

A partial display of kernel startup messages ..

sn0 at pica0 slot 4 offset 0x0:bufsize 2840 address 00:00:4c:75:2d:2a
sno: supplying EUI64: 00:00:4c:75:2d:2a
com0: ....
com1:..
....
boot device:<unknown>
root device:

The kernel pauses at root device...I need to do a 'diskless' boot
so I answer to the prompts as follows
root device:   sn0
dump device:  sn0
file system (default generic): nfs
The kernel responds with :
root on sn0 dumps of sn0p
nfs_boot: trying DHCP/BOOTP
sn0: starting DAD for fe80:0001::0200:4cff:fe75:2d2a

After the last message the kernel waits forever, the bootp
server never gets a request from the Ethernet address listed
above. Help !