HP A6600 Configuration Manual page 56

Class-based queuing
Figure 19 CBQ
Packets to be sent out
this interface
CBQ extends WFQ by supporting user-defined classes. When network congestion occurs, CBQ enqueues
packets by user-defined traffic classification rules. Before that, congestion avoidance actions, such as tail
drop or WRED and bandwidth restriction check, are performed before packets are enqueued. When being
dequeued, packets are scheduled by WFQ.
CBQ provides the following queues:
An emergency queue, which enqueues emergent packets. The emergency queue is a FIFO queue

without bandwidth restriction.
LLQ, which is an EF queue. Because packets are fairly treated in CBQ, delay sensitive flows like video

and voice packets may not be transmitted timely. To solve this problem, an EF queue was introduced to
preferentially transmit delay sensitive flows. LLQ combines PQ and CBQ to preferentially transmit delay
sensitive flows like voice packets. When defining traffic classes for LLQ, configure a class of packets to
be preferentially transmitted. Such a class is called a ―priority class.‖ The packets of all priority classes
are assigned to the same priority queue. Bandwidth restriction on each class of packets is checked
before the packets are enqueued. During the dequeuing operation, packets in the priority queue are
transmitted first. The other queues are scheduled by CBQ. To reduce the delay of the other queues
except the priority queue, LLQ assigns the maximum available bandwidth for each priority class. The
bandwidth value polices traffic during congestion. When no congestion is present, a priority class can
use more than the bandwidth assigned to it. During congestion, the packets of each priority class
exceeding the assigned bandwidth are discarded.
 BQ, which is an AF queue. The BQ provides strict, exact, guaranteed bandwidth for AF traffic, and
schedules the AF classes proportionally. The system supports up to 64 AF queues.
Default queue, which is a WFQ queue. It transmits the BE traffic by using the remaining interface

bandwidth.
Emergency queue
EF
AF 1
......
AF 64
Classify
Default queue
Packets sent
Schedule
50