3com 10014303 Manual De Usuario

As an extension of WFQ, class based queuing (CBQ) provides users with class 

definition support. CBQ assigns individual FIFO reservation queues to the classes 

defined by each user to buffer data of the same class. When there is network 

congestion, CBQ matches outbound packets according to the classification rule 

defined by users to make them enter relevant queues. Before queue entry of packets, 

the congestion avoidance mechanism (tail-drop or weighted random early detection 

[WRED]) and bandwidth limit must first be checked. When packets leave the queues, 

weighted fair scheduling of packets in the queues corresponding to each class should 

be performed. 

IP Packets

LLQ

BQ1

Classifying

Sent packets

BQ64

Sent queue

¡­ ¡­

Scheduling

BQ2

Outgoing first

 

Figure 1-1 CBQ diagram 

If CBQ performs weighted fair treatment to queues of all classes, voice packets, the 

delay-sensitive data flow may not be sent out in time. Therefore, PQ is introduced to 

CBQ to create low latency queuing (LLQ), which provides strictly preferred sending 

service for such delay-sensitive data flow as voice packets. 

LLQ strictly combines PQ with CBQ. When a user defines a class, he can specify it to 

accept strict priority service. The class of this type is called priority class. All packets 

of the priority class enter the same priority queue. Before they enter a queue, the 

bandwidth limit of each class of packets should be checked. When packets go out of 

the queues, the packets in the priority queue are forwarded before packets in the 

queues corresponding to other classes. But if the maximum reservation bandwidth 

configured for LLQ is exceeded, the packets in other queue are sent. Weighted fair 

scheduling will be performed to the packets in other queues when they are forwarded. 

In order to avoid long time delay of packets in other queues, the maximum available 

bandwidth can be specified for each priority class during LLQ application for traffic 

3Com Router Configuration Guide Addendum for V1.20

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