Limitations and Caveats
A notable difference between the priority class and other user-defined classes
under CBWFQ is that WRED is not available in the priority class. LLQ is to be
used for CBR services, especially VoIP.Voice traffic is UDP-based and therefore
not adaptive to the early packet drop characteristic of WRED. If a packet is
dropped from a UDP stream, UDP will not react to this by reducing the send
rate. Because WRED would be ineffective, configuration of this feature for a priority
class using the random-detect command is disallowed. Besides, would you
really want to randomly drop your voice traffic, anyway? Not likely.