Quality of Service (QoS) | 431
Strict-Priority Queueing
To assign strict-priority to one unicast queue, 1 to 3, use the strict-priority command from
CONFIGURATION mode. Strict-priority means that FTOS dequeues all packets from the assigned queue
before servicing any other queues.
• The
strict-priority command supersedes the bandwidth-percentage command 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.
Weighted Random Early Detection
The weighted random early detection (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 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 you can prevent specified traffic from consuming too much of the BTM resources.
WRED uses a profile to specify the 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
(Figure 24-11); 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 24-11. Packet Drop Rate for WREDl
You can create a custom WRED profile or use on of the five pre-defined profiles.
Min Max0KB
Buffer Space
Packet Drop Rate
0 Pckts
Total Buffer Spa
Allotted Space
Early Warning
No Packets Buffered
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