The ceil of the parent class is not respected IF the child classes asks for more. This can happen if the sum of the rates of the child classes is bigger ceil of the parent. Why? Performance reasons. Checking the parent ceil will create a lot of overhead and it's not so easy to implement.
For class 300, the parent ceil is 40. But if class 300 asks for 60, it will get 60. So why do you want to have class 20 a lower ceil then it's parent? It will never be respected.
Rule 2: sum (rate of childs) <= parent ceil
stef.coene@docum.org |
lartc.org FAQ : 
Ceil of a class