R3303-HP HSR6800 Routers ACL and QoS Configuration Guide

47
Priority queuing schedules the four queues in the descending order of priority. It sends packets in the
queue with the highest priority first. When the queue with the highest priority is empty, it sends packets
in the queue with the second highest priority. In this way, you can assign the mission-critical packets to
the high priority queue to make sure that they are always served first. The common service packets are
assigned to the low priority queues and transmitted when the high priority queues are empty.
The disadvantage of priority queuing is that packets in the lower priority queues cannot be transmitted
if packets exist in the higher queues for a long time when congestion occurs.
CQ
Figure 18 Custom queuing (CQ)
CQ provides 17 queues, numbered from 0 to 16. Queue 0 is a reserved system queue, and queues 1
through 16 are customer queues, as shown in Figure 18.
You can define traffic classification rules and
assign a percentage of interface/PVC bandwidth for each customer queue. By default, packets are
assigned to queue 1.
During a cycle of queue scheduling, CQ first empties the system queue. Then, it schedules the 16 queues
in a round robin way: it sends a certain number of packets (based on the percentage of interface
bandwidth assigned for each queue) out of each queue in the ascending order of queue 1 to queue 16.
CQ guarantees normal packets a certain amount of bandwidth, and ensures that mission-critical packets
are assigned more bandwidth.
CQ can assign free bandwidth of idle queues to busy queues. Even though it performs round robin queue
scheduling, CQ does no assign fixed time slots for the queues. If a queue is empty, CQ immediately
moves to the next queue. When a class does not have packets, the bandwidth for other classes increases.