It was intended that non-backbone  sites may use mail-via=DISTRIBUTE, but this
is not recommended unless their various tables are up to date. With DIST2, the
source node will  decide the exact path  to be followed by the  files and will
enforce  it  upon  the other  servers.  If  it  chooses  a stupid  path,  mail
distribution will be inefficient and it  won't be the backbone's fault. Anyway
DIST2 is not yet in use so this is a bit academic.
 
  Eric