Deployment Guide

Configuring Policy-Based Rate Shaping
You can configure the rate shaping for QoS output policies in packets per second (pps).
You can explicitly specify the rate shaping functionality for QoS output policies as peak rate and committed rate attributes. You
can also configure the peak burst and committed burst sizes. All of these settings can be configured in Kbps, Mbps, or pps.
To configure the peak and committed rates and burst sizes, perform the following steps:
1. Configure the peak rate and peak burst size in pps in QoS Policy Out Configuration mode.
QOS-POLICY-OUT mode
DellEMC(config-qos-policy-out)# rate shape pps peak-rate burst-packets
2. Alternatively, configure the peak rate and peak burst size in bytes.
QOS-POLICY-OUT mode
DellEMC(config-qos-policy-out)# rate shape Kbps peak-rate burst-KB
3. Configure the committed rate and committed burst size in pps.
QOS-POLICY-OUT mode
DellEMC(config-qos-policy-out)# rate shape pps peak-rate burst-packets committed pps
committed-rate burst-packets
4. Alternatively, configure the committed rate and committed burst size in bytes.
QOS-POLICY-OUT mode
DellEMC(config-qos-policy-out)# rate shape Kbps peak-rate burst-KB committed Kbps
committed-rate burst-KB
Configuring Weights and ECN for WRED
The WRED congestion avoidance functionality drops packets to prevent buffering resources from being consumed. Traffic is a
mixture of various kinds of packets. The rate at which some types of packets arrive might be greater than others. In this case,
the space on the buffer and traffic manager (BTM) (ingress or egress) can be consumed by only one or few types of traffic,
leaving no space for other types. You can apply a WRED profile to a policy-map so that the specified traffic can be prevented
from consuming too much of the BTM resources.
WRED drops packets when the average queue length exceeds the configured threshold value to signify congestion. ECN is a
capability that enhances WRED by marking the packets instead of causing WRED to drop them when the threshold value is
exceeded. If you configure ECN for WRED, devices employ ECN to mark the packets and reduce the rate of sending packets in
a congested network.
In a best-effort network topology, data packets are transmitted in a manner in which latency or throughput is not maintained
to be at an effective level. Packets are dropped when the network experiences a large traffic load. This best-effort network
deployment is not suitable for applications that are time-sensitive, such as video on demand (VoD) or voice over IP (VoIP)
applications. In such cases, you can use ECN in conjunction with WRED to resolve the dropping of packets under congested
conditions.
Using ECN, the packets are marked for transmission at a later time after the network recovers from the heavy traffic state
to an optimal load. In this manner, enhanced performance and throughput are achieved. Also, the devices can respond to
congestion before a queue overflows and packets are dropped, enabling improved queue management.
When a packet reaches the device with ECN enabled for WRED, the average queue size is computed. To measure the average
queue size, a weight factor is used. This weight factor is user-configurable. You can use the wred weight number command
to configure the weight for the WRED average queue size. The mark probability value is the number of packets dropped when
the average queue size reaches the maximum threshold value.
The weight factor is set to zero by default, which causes the same behavior as dropping of packets by WRED during network
loads or also called instantaneous ECN marking. In a topology in which congestion of the network varies over time, you can
specify a weight to enable a smooth, seamless averaging of packets to handle the sudden overload of packets based on the
previous time sampling performed. You can specify the weight parameter for front-end and backplane ports separately in the
range of 0 through 15.
You can enable WRED and ECN capabilities per queue for granularity. You can disable these functionality per queue, and you
can also specify the minimum and maximum buffer thresholds for each color-coding of the packets. You can configure maximum
drop rate percentage of yellow and green profiles. You can set up these parameters for both front-end and backplane ports.
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Quality of Service (QoS)