Traffic Management - CCIE Routing and Switching Exam Certification Guide - page 528

WRR

WRR is enabled by default on Cisco Layer-3 switches (Cisco 8500 or 6500 switches) egress
ports. With WRR, the administrator uses IP Precedence bits to configure policies for traffic. IP
Precedence bits are set at a device other than the switch that is using WRR. As packets enter the
Layer-3 switch, WRR maps packets that are using IP Precedence bits to one of four outbound
WRR queues. This is also known as WRR scheduling . Each WRR queue has a queue weight and
delay priority. More bandwidth is given to packets with higher weight. Table 11-3 shows IP
Precedence to WRR queue assignments.

IP Precedence to WRR Queue Assignments Table 11-3

IP
Precedence

WRR Queue
Assigned

WRR Queue
Weight

WRR Delay
Priority

0,1 0 1 0

2,3 1 2 1

4,5 2 4 2

6,7 3 8 3

RED and WRED
RED is a congestion avoidance mechanism that randomly drops packets before congestion can
occur. RED uses TCP's congestion control mechanisms by dropping packets and letting TCP
reduce the source host's window size. RED is typically implemented in the core of the network
on IP networks. The disadvantage of RED is that dropped packets can affect UDP or Novell
Internetwork Packet Exchange (IPX) transmissions that do not implement a windowing flow
control mechanism.

WRED is a Cisco implementation of RED that implements a preferential treatment of packets
when determining which packets to drop when congestion occurs. WRED uses the IP Prece-
dence bits to determine which packets to drop. The higher the IP Precedence is in a packet, the
less likely the packet might be dropped. Up to six CoS can be configured.

WRED can also be configured to use other factors. WRED drops packets that are not part of an
RSVP flow on interfaces that are configured for RSVP. WRED can also be configured to use
Differentiated Services Code Point (DSCP) values as the decision factor of packets to be
dropped. DSCP is explained in the following subsection.

WRED is configured with the following interface command, which uses default parameters:

interface interface num
random-detect
Where interface num is the interface name and number (such as serial 0).