Reference Guide

Enabling QoS Rate Adjustment
By default, while rate limiting, policing, and shaping, FTOS 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.
The range is from 1 to 31.
Enabling Strict-Priority Queueing
Strict-priority means that FTOS 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.
• Assign strict priority to one unicast queue.
CONFIGURATION mode
strict-priority
The range is from 1 to 7.
Weighted Random Early Detection
Weighted random early detection (WRED) is supported on the S4820T platform.
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
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