Congestion Management - Huawei Quidway S7700 Configuration Manual - Qos

Quidway S7700 Smart Routing Switch
Configuration Guide - QoS
the drop policy based on colors of packets, so the drop probability of packets with higher
priorities is low.

3.1.2 Congestion Management

When intermittent congestion occurs on the network, delay-sensitive services require higher
QoS than others. In this case, congestion management is required. The bandwidth needs to be
increased if a network is always congested.
Congestion management uses the queue scheduling technologies. Currently, the S7700 adopts
the following queue scheduling modes:
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PQ Scheduling
Priority Queuing (PQ) scheduling is a queuing technology by which packets are scheduled based
on the priorities of queues in a strict manner. The packets of lower priorities can be scheduled
only after packets of higher priorities are scheduled.
In PQ scheduling mode, packets of delay-sensitive core services are put into a high priority queue
and packets of other non-core services are put into a low priority queue. This ensures that core
services are sent first.
The disadvantage of PQ scheduling is that the packets of lower priorities are not processed if
there are a large number of packets of higher priorities, when congestion occurs.
WRR Scheduling
WRR refers to Weighted Round Robin. WRR schedules packets of queues in a polling manner,
ensuring that packets in each queue are sent at a certain time.
Assume that there are eight output queues on an interface. WRR sets weights for the eight queues,
that is, w7, w6, w5, w4, w3, w2, w1, and w0. The weight indicates a percentage of obtaining
resources. For example, the weights of queues on a 100-Mbit/s interface are set to 50, 50, 30,
30, 10, 10, 10, and 10, corresponding to w7, w6, w5, w4, w3, w2, w1, and w0. In this case, the
lowest priority queue can obtain bandwidth of at least 5 Mbit/s. This avoids the disadvantage of
PQ scheduling.
The advantage of WRR is as follows: Although packets in multiple queues are processed in a
polling manner, the time allocated to each queue is not fixed. If a queue is null, packets of the
next queue are scheduled. This ensures better usage of bandwidth.
The disadvantages of WRR are as follows:
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PQ Scheduling
WRR Scheduling
DRR Scheduling
PQ+WRR/PQ+DRR Scheduling
WRR allocates bandwidth according to the number of packets. When the average length
of packets in each queue is the same or known, you can obtain the required bandwidth by
setting the weight of WRR. You, however, cannot obtain the required bandwidth by setting
the weight of WRR when the average length of packets in each queue changes.
The packets of short-delay services such as voice services cannot be scheduled in time.
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3 Congestion Avoidance and Congestion Management
Configuration
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