Dell S6000-ON, Configuration Manual

Page: 731 / 1080

4. Alternatively, configure the committed rate and committed burst size in bytes.
QOS-POLICY-OUT mode
Dell(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.
Quality of Service (QoS)
731