In my previous post of QOS I tried to describe the basic qos models. Dennis Hartman is already doing a great job on qos and the way he is explaining a fair enough to understand the basic concept of CBWFQ.
In this post, I only want to add the few common and unforgetable points which should be learn by heart for each and every technology guy.

Facts Of CBWFQ
1. 64 number of clases can be configured and by default per class allocates 64 packets in queue.
2. Trafic shaping and Policing is not supported by CBWFQ.
3. If wred is using under policy map then it cannot be added under interface.
(WRED is weighted random early detection)
4. CBWFQ is not supported on sub interfaces.
5. Default class uses the fair queue mechanism. If any other mechanism is implemented it is overriden.