H3C Technologies H3C SecPath F1000-E - Manual (Page 514)

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Because WFQ can balance the delay and jitter of each flow when congestion occurs, it is suitable for
handling some special occasions. For example, WFQ is used in the assured forwarding (AF) services of
the Resource Reservation Protocol (RSVP). In Generic Traffic Shaping (GTS), WFQ is used to schedule
buffered packets.
CBQ
Class-based queuing (CBQ) extends WFQ by supporting user-defined classes. CBQ assigns an
independent reserved FIFO queue for each user-defined class to buffer data of the class. When network
congestion occurs, CBQ enqueues packets by user-defined traffic classification rules. Before that,
congestion avoidance actions such as tail drop or weighted random early detection (WRED) and
bandwidth restriction check are performed. When being dequeued, packets are scheduled by WFQ.
CBQ provides an emergency queue to enqueue emergent packets. The emergency queue is a FIFO
queue without bandwidth restriction. However, realtime packets (such as voice and video packets, which
are delay-sensitive) may not be transmitted timely in CBQ since packets are fairly treated. To solve this
issue, Low Latency Queuing (LLQ) was introduced to transmit realtime packets preferentially.
When defining traffic classes for LLQ, you can configure a class of packets to be transmitted preferentially.
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. WFQ is used to
dequeue packets in the other queues.
In order 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 is used to police 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. LLQ can also specify burst-size.
The system matches packets with classification rules in the following order:
• Match packets with priority classes and then the other classes.
• Match packets with priority classes in the configuration order.
• Match packets with other classes in the configuration order.
• Match packets with classification rules in a class in the configuration order.
Congestion management technology comparison
The following table compares these queuing technologies for efficient use.