I would 

1) run tcpdump and see if the RS packets are thought to be ok
2) read the rtadvd code, and turn on debugging or add printfs for why
   RS are rdjected
3) run gdb on rtadvd and see what it does with the RS packet
4) Check the host against some other non-NetBSD RA source
5) Try quagga, which has router advertisment facilities
-- 
        Greg Troxel <gdt@ir.bbn.com>