Reference Guide

678 | Quality of Service (QoS)
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Strict-priority Queueing
You can assign strict-priority to one unicast queue, using the strict-priority command from
CONFIGURATION mode. Strict-priority means that FTOS dequeues all packets from the assigned queue
before servicing any other queues.
Policy-based Per queue rate shaping does not take effect on the queue configured with strict-priority
unicast <Queue>. To achieve queue-based rate-shaping as well as “strict-priority” treatment for a
particular queue, use the Scheduler Strict CLI present under Policy-based QoS.
• The
strict-priority configurations supersedes bandwidth-percentage configurations.
• A queue with strict-priority can starve other queues in the same port-pipe.
Weighted Random Early Detection
Weighted Random Early Detection is supported only on the platforms z
Weighted random early detection (WRED) congestion avoidance mechanism that 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 is 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 a linear rate until the maximum threshold is reached (Figure 37-8); this is
the “early detection” part of WRED. If the maximum threshold—2000KB, for example—is reached, all
incoming packets are dropped until less than 2000KB of buffer space is consumed by the specified traffic.
Figure 37-8. Packet Drop Rate for WRED