Benefits Of Using Weighted Random Early Detection; How Wred Works; Wred Drop Mode - Cisco 10000 Series Configuration Manual

Chapter 11 Managing Packet Queue Congestion

Benefits of Using Weighted Random Early Detection

Weighted random early detection (WRED) has the following benefits:
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How WRED Works

When an output interface begins to show signs of congestion, but is not fully congested yet, WRED
begins selectively dropping packets based on the IP precedence, DSCP, or discard-class level of the
packet. WRED uses the minimum and maximum threshold values to determine the drop probability for
packets. Typically, for drop policies with low values (closer to 0), WRED drops packets with low values
(closer to 0) with a higher probability, based on the drop policy configuration.
For example, IP precedence-based WRED uses the IP precedence level of packets to selectively drop
packets. WRED drops packets with a low precedence value (closer to 0) with a higher probability based
on the WRED configuration. The higher the precedence value of a packet (the closer to 7), the greater
the probability that WRED ignores the packet and allows the router to forward the packet to its
destination.
WRED allows the transmission line to be used fully at all times, especially when most of the traffic is
TCP/IP traffic. For TCP/IP traffic, the action of WRED dropping packets indicates congestion and
causes a source host to reduce its transmission rate. After all of the packets reach their destinations
(indicating reduced congestion), the source host increases the transmission rate again.
By dropping some packets early, WRED avoids dropping large numbers of packets at once and
minimizes the chances of global synchronization. Global synchronization occurs when multiple TCP
hosts reduce their transmission rates in response to packet dropping and then increase their transmission
rates again when congestion reduces.
With protocols other than TCP/IP, packet sources might not respond to dropped packets by reducing their
transmission rates. Instead, such hosts might resend the dropped packets at the same rate. As a result,
when WRED drops packets, congestion is not decreased.
Statistically, WRED drops more packets from large users than from small users. Therefore, WRED is
more likely to slow down the traffic sources that generate the most traffic and not the traffic sources that
generate little traffic.

WRED Drop Mode

A WRED drop mode indicates the basis upon which the WRED drop policy is applied. The router
supports DSCP, IP precedence, discard-class, and ATM cell loss priority (CLP) based WRED.
Table 11-4
If you do not specify any arguments, WRED uses the default IP precedence in the WRED calculations.
Note
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WRED provides early detection of congestion for one or multiple traffic classes. It also protects
against global synchronization. For these reasons, WRED is useful on any outbound interface where
you expect congestion to occur.
WRED provides separate thresholds and weights for different IP precedence levels, which allows
you to provide different qualities of service for packet dropping for different traffic types. For
example, during congestion WRED can drop standard traffic more frequently than premium traffic.
lists the commands used to enable the WRED drop modes.
Controlling Congestion Using Weighted Random Early Detection
Cisco 10000 Series Router Quality of Service Configuration Guide
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