Congestion Avoidance And Queuing - Cisco 3845 - Security Bundle Router Software Manual

Chapter 27 Configuring QoS
You specify and mark traffic by using the set commands in a policy map for all supported QoS markings
(CoS, IP DSCP, IP precedence, and QoS groups). A set command unconditionally marks the packets that
match a specific class. You then attach the policy map to an interface or service instance as an input
policy map.
You can simultaneously configure actions to modify DSCP, precedence, and COS markings in the packet
for the same service along with QoS group marking actions. You can use the QoS group number defined
in the marking action for egress classification.
Figure 27-6
This example uses a policy map to remark a packet. The first marking (the set command) applies to the
QoS default class map that matches all traffic not matched by class AF31-AF33 and sets all traffic to an
IP DSCP value of 1. The second marking sets the traffic in classes AF31 to AF33 to an IP DSCP of 3.
Switch(config)# policy-map Example
Switch(config-pmap)# class class-default
Switch(config-pmap-c)# set ip dscp 1
Switch(config-pmap-c)# exit
Switch(config-pmap)# class AF31-AF33
Switch(config-pmap-c)# set ip dscp 3
Switch(config-pmap-c)# exit
Switch(config-pmap)# exit
Switch(config)# interface gigabitethernet0/1
Switch(config-if)# service-policy input Example
Switch(config-if)# exit

Congestion Avoidance and Queuing

Congestion avoidance uses algorithms such as tail drop to control the number of packets entering the
queuing and scheduling stage to avoid congestion and network bottlenecks. The switch uses WTD to
manage the queue sizes and provide a drop precedence for traffic classifications. You set the queue size
limits depending on the markings of the packets in the queue. You can assign each packet that travels
through the switch to a specific queue and threshold. For example, you can map specific DSCP or CoS
values to a specific egress queue and threshold. If the total destination queue size is greater than the
threshold of any classified traffic, the next frame of that traffic is dropped.
Figure 27-7
Three drop percentages are configured: 40 percent (400 microseconds), 60 percent (600 microseconds),
and 100 percent (1000 microseconds). These percentages mean that traffic classified to the 40-percent
threshold is dropped when the queue depth exceeds 400 microseconds, traffic classified to 60 percent is
dropped when the queue depth exceeds 600 microseconds. Traffic up to 400 microseconds can be queued
at the 40-percent threshold, up to 600 microseconds at the 60-percent threshold, and up to 1000
microseconds at the 100-percent threshold.
OL-23400-01
Marking of Classified Traffic
Receive
Classify
shows an example of WTD operating on a queue with 1000 microseconds worth of data.
Figure 27-6 shows the steps for marking traffic.
Unconditionally
mark traffic for
reclassification
Cisco ME 3800X and 3600X Switch Software Configuration Guide
Understanding QoS
Queuing,
scheduling,
and shaping
27-15