Administrator Guide

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Enabling QoS Rate Adjustment
By default while rate limiting, policing, and shaping, Dell EMC Networking OS does not include the Preamble, SFD, or the IFG fields. These
fields are overhead; only the fields from MAC destination address to the CRC are used for forwarding and are included in these rate
metering calculations.
The Ethernet packet format consists of:
• Preamble: 7 bytes Preamble
• Start frame delimiter (SFD): 1 byte
• Destination MAC address: 6 bytes
• Source MAC address: 6 bytes
• Ethernet Type/Length: 2 bytes
• Payload: (variable)
• Cyclic redundancy check (CRC): 4 bytes
• Inter-frame gap (IFG): (variable)
You can optionally include overhead fields in rate metering calculations by enabling QoS rate adjustment.
QoS rate adjustment is disabled by default.
• Specify the number of bytes of packet overhead to include in rate limiting, policing, and shaping calculations.
CONFIGURATION mode
qos-rate-adjust overhead-bytes
For example, to include the Preamble and SFD, type qos-rate-adjust 8. For variable length overhead fields, know the number of
bytes you want to include.
The default is disabled.
The range is from 1 to 31.
Enabling Strict-Priority Queueing
In strict-priority queuing, the system de-queues all packets from the assigned queue before servicing any other queues. You can assign
strict-priority to one unicast queue, using the strict-priority command.
• Policy-based per-queue rate shaping is not supported on the queue configured for strict-priority queuing. To use queue-based rate-
shaping as well as strict-priority queuing at the same time on a queue, use the Scheduler Strict feature as described in Scheduler
Strict .
• The strict-priority supersedes bandwidth-percentage configuration.
• A queue with strict priority can starve other queues in the same port-pipe.
• Assign strict priority to one unicast queue.
INTERFACE mode
service-policy output policy-map-name
Enter the name for the policy map in character format (32 characters maximum).
Weighted Random Early Detection
Weighted random early detection (WRED) is a congestion avoidance mechanism that drops packets to prevent buffering resources from
being consumed.
The WRED congestion avoidance mechanism 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 a few types of traffic, leaving no
space for other types. You can apply a WRED profile to a policy-map so that specified traffic can be prevented from consuming too much
of the BTM resources.
WRED uses a profile to specify minimum and maximum threshold values. The minimum threshold is the allotted buffer space for specified
traffic, for example, 1000KB on egress. If the 1000KB is consumed, packets are dropped randomly at an exponential rate until the
maximum threshold is reached (as shown in the following illustration); this procedure is the “early detection” part of WRED. If the
Quality of Service (QoS)
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