White Papers

Enabling QoS Rate Adjustment
By default, while rate limiting, policing, and shaping, the Dell 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, and no qos-rate-adjust is listed in the running-configuration
● Include a specified 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, enter qos-rate-adjust 8. For variable length overhead fields, know the
number of bytes you want to include.
The default is disabled.
Enabling Strict-Priority Queueing
Strict-priority means that the Dell Networking OS de-queues all packets from the assigned queue before servicing any other
queues.
● The strict-priority supersedes bandwidth-percentage and bandwidth-weight percentage configurations.
● A queue with strict priority can starve other queues in the same port-pipe.
● If more than two strict priority queues are configured, the strict priority queue with a higher queue number is scheduled first.
● Assign strict priority to one unicast queue.
CONFIGURATION mode
strict-priority
The range is from 1 to 3.
Weighted Random Early Detection
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 maximum threshold, for example, 2000KB, is reached, all incoming packets are dropped until the buffer space
consumes less than 2000KB of the specified traffic.
Quality of Service (QoS)
627