You will need a system of your choice to boot your Indigo2 over network because it's not possible to boot it directly at the moment. You can use any arch which has the necessary packages ported. But I advise to use either an Alpha, Sparc or Intel system because these ports are pretty stable these days.. You need to install a BOOTP server on it and an NFS server. The BOOTP server will send the Kernel to your Indigo2 using TFTP (so you also need a TFTP server). Then the kernel will contact the NFS server and mount a NFS volume as root device.  
 
Je sais pas si ça répond à ta question, je connais mal les SGI