Users Guide

• 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 elds 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 elds, 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 congured 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 conguration.
• 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 3.
Weighted Random Early Detection
Weighted random early detection (WRED) is a congestion avoidance mechanism that drops packets to prevent buering resources from
being consumed.
The WRED congestion avoidance mechanism drops packets to prevent buering resources from being consumed.
Trac 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 buer and trac manager (BTM) (ingress or egress) can be consumed by only one or a few types of trac, leaving no
space for other types. You can apply a WRED prole to a policy-map so that specied trac can be prevented from consuming too much
of the BTM resources.
WRED uses a prole to specify minimum and maximum threshold values. The minimum threshold is the allotted buer space for specied
trac, 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,
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Quality of Service (QoS)