R2511-HP MSR Router Series ACL and QoS Configuration Guide(V5)

CBQ

Figure 18 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 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:

• 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, 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, you can 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. 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.

• Bandwidth queuing (BQ)—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—A WFQ queue. It transmits the BE traffic by using the remaining interface

bandwidth.

The system matches packets with classification rules in the following order:

• Match packets with priority classes and then the other classes.