We have a number of CPU servers and file servers.  We run the auth server
on a kfs machine with a UPS.  That machine serves bootp/dhcp/tftp, authentication, and
consolefs.  It gives us a stable point in the network.  While kfs isn't
the most robust software, it's stable enough for something with a limited
function.  It gives us someplace to stand while repairing the world when
catastrophe strikes.  I also keep a copy of the the key files and kernels
and /lib/ndb on the file server so that we can rebuild the auth server
should it ever blow up.