4000 Router Series ACL and QoS Configuration Guide

WFQ considers weights when determining the queue scheduling order. Statistically, WFQ gives

high-priority traffic more scheduling opportunities than low-priority traffic. WFQ automatically classifies

traffic according to the "session" information of traffic (for example, protocol type, TCP or UDP

source/destination port numbers, source/destination IP addresses, IP precedence bits in the ToS field),

and attempts to provide as many queues as possible so that each traffic flow can be put into these queues

to balance the delay of every traffic flow. When dequeuing packets, WFQ assigns the outgoing interface

bandwidth to each traffic flow by precedence. The higher precedence value a traffic flow has, the more

bandwidth it gets.

For example, five flows exist in the current interface with precedence 0, 1, 2, 3, and 4, respectively. The

total bandwidth quota is the sum of all the (precedence value + 1)s, 1 + 2 + 3 + 4 + 5 = 15.

The bandwidth percentage assigned to each flow is (precedence value of the flow + 1)/total bandwidth

quota. The bandwidth percentages for flows are 1/15, 2/15, 3/15, 4/15, and 5/15, respectively.

Because WFQ can balance the delay and jitter of each flow when congestion occurs, it is suitable for

handling special situations. For example, WFQ is used in the assured forwarding (AF) services of the

RSVP, and WFQ is used to schedule buffered packets in GTS.

CBQ

Figure 16 CBQ

Class-based queuing (CBQ) extends WFQ by supporting user-defined classes. When network

congestion occurs, CBQ uses user-defined traffic match criteria to enqueue packets. Before packets are

enqueued, congestion avoidance actions, such as tail drop or WRED and bandwidth restriction check,

are performed. When being dequeued, packets are scheduled by WFQ.

CBQ provides the following queues:

• Emergency queue—Enqueues emergent packets. The emergency queue is a FIFO queue without

bandwidth restriction.

• Low Latency Queuing (LLQ)—An EF queue. Because packets are fairly treated in CBQ,

delay-sensitive flows like video and voice packets might not be transmitted timely. To solve this

problem, LLQ combines PQ and CBQ to preferentially transmit delay-sensitive flows like voice