Queue bandwidth preference

----MASTER_CLASS (LIMIT 45)
  |
  |-----> SUB_CLASS_ONE (35)
      |
      |-----> CHILD_ONE (LIMIT 10/30)
      |
      |-----> CHILD_TWO (LIMIT 5/20)

Have a configuration with hierarchical queue structure as shown above. I would like to be able to provide a method whereby CHILD_TWO can hit a maximum level up to 20mb only if CHILD_ONE is not using all of its available bandwidth, including the maximum levels. CHILD_ONE should have preference on the available maximum bandwidth if required. The ‘preference’ marker seems only to be used when one is dealing with rate_limit settings that provide a guaranteed bandwidth amount and not maximum levels.

Can this be done using simple queues?

Regs.

Iain.