(i.e. you get * * *) 
Actually it doesn't quite work like that as in a traceroute you always
see the return i/f, so on a traceroute from A to B to C you see
routerb-x-y-z-routera.blah.net
routerc-x-y-z-routerb.blah.net
It's mainly useful because you immediately know that a link is OK,
if you just se serial0 or whatever with equipment that sometimes
skips hop reports and sometimes double reports, sometimes is in
mid flap, it saves working out whether that was the "right" serial
i/f.
Alex Bligh
Xara Networks