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Catalyst 3550 Multilayer Switch Software Configuration Guide
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Chapter 20 Configuring QoS
Understanding QoS
WRED
Cisco’s implementation of Random Early Detection (RED), called Weighted Random Early Detection
(WRED), differs from other congestion-avoidance techniques because it attempts to anticipate and avoid
congestion, rather than controlling congestion once it occurs.
WRED takes advantage of TCP congestion control to try to control the average queue size by indicating
to end hosts when they should temporarily stop sending packets. By randomly dropping packets before
periods of high congestion, it tells the packet source to decrease its transmission rate. Assuming the
packet source is using TCP, WRED tells it to decrease its transmission rate until all the packets reach
their destination, meaning that the congestion is cleared.
WRED reduces the chances of tail drop by selectively dropping packets when the output interface begins
to show signs of congestion. By dropping some packets early rather than waiting until the queue is full,
WRED avoids dropping large numbers of packets at once. Thus, WRED allows the transmission line to
be fully used at all times. WRED also drops more packets from large users than small. Therefore, traffic
sources that generate the most traffic are more likely to be slowed down than traffic sources that generate
little traffic.
You can enable WRED and configure the two threshold percentages assigned to the four egress queues
on a Gigabit-capable Ethernet port by using the wrr-queue random-detect max-threshold interface
configuration command. Each threshold percentage represents where WRED starts to randomly drop
packets. After a threshold is exceeded, WRED randomly begins to drop packets assigned to this
threshold. As the queue limit is approached, WRED continues to drop more and more packets. When the
queue limit is reached, WRED drops all packets assigned to the threshold. By default, WRED is
disabled.
You modify the DSCP-to-threshold map to determine which DSCPs are mapped to which threshold ID
by using the wrr-queue dscp-map interface configuration command. By default, all DSCPs are mapped
to threshold 1, and when this threshold is exceeded, all the packets are randomly dropped.
If you use WRED thresholds, you cannot use tail drop, and vice versa. If WRED is disabled, tail drop is
automatically enabled with the previous configuration (or the default if it was not previously
configured).
