Reference Guide

740 | Quality of Service (QoS)
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• 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 S5000 switch.
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 BTM (ingress or egress) can be consumed by only one or
a few types of traffic, leaving no space for other types. A WRED profile can be applied 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 will be dropped randomly at an exponential rate until the maximum threshold is reached
(Figure 39-10); this is the “early detection” part of WRED. If the maximum threshold—2000KB, for
example—is reached, then all incoming packets are dropped until less than 2000KB of buffer space is
consumed by the specified traffic.
Figure 39-10. Packet Drop Rate for WREDl
You can create a custom WRED profile or use on of the five pre-defined profiles.
Table 39-6. Pre-defined WRED Profiles for the S5000
Default Profile
Name
Minimum
Threshold
Maximum
Threshold
Maximum
Drop Rate
wred_drop 0 0 100
wred_teng_y 467 4671 100
wred_teng_g 467 4671 50
Min Max0KB
Buffer Space
Packet Drop Rate
0 Pckts
All Pckts
Total Buffer Space
Allotted Space
Early Warning
No Packets Buffered
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