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Catalyst 3750 Metro Switch Software Configuration Guide
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Chapter 26 Configuring QoS
Configuring Hierarchical QoS
discards or marks packets destined for a queue when that queue is congested. It discards packets fairly
and before the queue is full. Packets with high IP-precedence values are preferred over packets with low
IP-precedence values. If you want to enable DSCP-based WRED instead of IP precedence-based WRED,
see the “Configuring CBWFQ and DSCP-Based WRED” section on page 26-89
Before beginning this procedure, make sure that you have reviewed the configuration guidelines and
have created the class map to isolate traffic. For more information, see the “Hierarchical QoS
Configuration Guidelines” section on page 26-76 and the “Classifying Egress Traffic by Using Class
Maps” section on page 26-78. For information on how to configure tail drop, see the “Configuring
CBWFQ and Tail Drop” section on page 26-86.
Beginning in privileged EXEC mode, follow these steps to configure class-level CBWFQ and IP
precedence-based WRED in a service policy for outbound traffic. This procedure is optional. The
examples that follow the procedure show how to configure class-level and VLAN-level CBWFQ and IP
precedence-based WRED.
Command Purpose
Step 1
configure terminal Enter global configuration mode.
Step 2
policy-map policy-map-name Create a policy map by entering the policy-map name, and enter
policy-map configuration mode.
By default, no policy maps are defined.
Step 3
class class-name Specify the name of the class whose traffic policy you want to create
or change, and enter policy-map class configuration mode.
By default, no traffic classes are defined.
Step 4
bandwidth {bandwidth-kbps | percent
percent}
Specify the minimum bandwidth provided to a class belonging to the
egress policy-map when there is traffic congestion in the switch. If the
switch is not congested, the class receives more bandwidth than you
specify with the bandwidth command.
CBWFQ derives the weight for packets belonging to the class from the
bandwidth allocated to the class. CBWFQ then uses the weight to
ensure that the queue for the class is serviced fairly.
By default, no bandwidth is specified.
You can specify the bandwidth in kbps or as a percentage:
• For bandwidth-kbps, specify the bandwidth amount in kbps
assigned to the class. The range is 200 to 2000000. Allocate the
bandwidth in 100-kbps increments; otherwise, the software rounds
down the bandwidth to the nearest 100-kbps increment.
• For percent percent, specify the percentage of available
bandwidth assigned to the class. The range is 1 to 100. The sum of
the class bandwidth percentages within a single policy map cannot
exceed 99 percent. Percentage calculations are based on the
bandwidth available at the parent class (or the physical level if it
is the parent).
Specify all the class bandwidths in either kbps or in percentages, but
not a mix of both. The amount of bandwidth configured should be large
enough to accommodate Layer 2 overhead.
Step 5
random-detect precedence-based Enable IP precedence-based WRED as a drop policy. By default,
WRED is disabled.
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