Management Guide
CLI
TigerAccess™ EE
SMC7824M/VSW
203
7.6.9.7
Weighted Random Early Detection (WRED)
The switch supports Weighted Random Early Detection (WRED) which can selectively
discard lower priority traffic when the interface begins to get congested and provide dif-
ferentiated performance characteristics for different classes of service. It minimizes the
impact of dropping high priority traffic. WRED is based on the RED algorithm.
RED, which utilizes end-to-end flow-control of TCP, is a random packet dropping function
when traffic reaches the user-designated threshold even before it reaches maximum
buffer size. If traffic usage reaches maximum buffer size, all packets can be dropped,
which makes packet loss. Therefore, in order to prevent packet loss or unstable traffic
transmission, user can restrict excessive traffic over buffer size by setting up a threshold.
With RED function, packet loss is reduced and stable packet transmission can be ac-
quired.
One of the drawbacks to implement RED function is that it randomly drops large numbers
of packets, and easy to drop high priority of packets. Unlike RED, WRED is not as ran-
dom when dropping packets. WRED combines the capabilities of the RED algorithm with
the IP precedence feature to provide for preferential traffic handling of high-priority pack-
ets.
To utilize WRED function, start queue length value, end queue length value and drop
probability are necessary. Start queue length represents the starting point of random
packet dropping, and drop probability indicates the percentage of packet dropping from
the starting point of random packet dropping to the point of complete dropping. If probabil-
ity is large value, large amount of packets would be dropped. Therefore complete drop-
ping point is slowly reached. On the other hand, if probability is little, little amount of
packets would be dropped. Therefore complete dropping point is quickly reached. If the
probability value is 1, dropping packet would be none and the value is 15, all packets
would be discarded from the point of start queue length value is reached.
Start
End
Queue Length
Drop
100%
Probability
Fig. 7.13
WRED Packet Drop Probability
In creating WRED profile, you can determine how to treat different types of traffic and as-
sign packets with certain values to specific threshold via queue numbers. Additionally,
WRED profile is specified to each port.
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