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Catalyst 3750 Metro Switch Software Configuration Guide
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Chapter 26 Configuring QoS
Configuring Hierarchical QoS
This example shows how to configure a VLAN-level policy called parent. It matches packets with VLAN
101 and associates a class-level child policy called policy1. The child policy matches two DSCP values
in two classes. Thirty percentage of the available bandwidth is assigned to the gold class, 20 percent is
assigned to the silver class, and 20 percent is assigned to vlan101. DSCP-based WRED is used as the
drop policy. For the af11 value in the gold class, WRED randomly drops packets with this DSCP when
the minimum threshold reaches 30. When the average queue size exceeds the maximum threshold of 40,
WRED drops all packets with DSCP af11. The mark-probability denominator is set to 10, which means
that one out of every 10 packets is dropped when the average queue is at the maximum threshold. The
configuration has similar settings for af12 in the gold class and for af21 and af22 in the silver class. Note
that when you configure the bandwidth command in a class policy, you also must configure the
bandwidth or shape policy-map class configuration command in the parent VLAN-level policy.
Switch(config)# class-map gold
Switch(config-cmap)# match ip dscp af11 af12
Switch(config-cmap)# exit
Switch(config)# class-map silver
Switch(config-cmap)# match ip dscp af21 af22
Switch(config-cmap)# exit
Switch(config)# class-map vlan101
Switch(config-cmap)# match vlan 101
Switch(config-cmap)# exit
Switch(config)# policy-map policy1
Switch(config-pmap)# class gold
Switch(config-pmap-c)# bandwidth percent 30
Switch(config-pmap-c)# random-detect dscp-based
Switch(config-pmap-c)# random-detect dscp af11 30 40 10
Switch(config-pmap-c)# random-detect dscp af12 25 40 10
Switch(config-pmap-c)# exit
Switch(config-pmap)# class silver
Switch(config-pmap-c)# bandwidth percent 20
Switch(config-pmap-c)# random-detect dscp-based
Switch(config-pmap-c)# random-detect dscp af21 28 35 10
Switch(config-pmap-c)# random-detect dscp af22 26 35 10
Switch(config-pmap-c)# exit
Switch(config-pmap)# exit
Switch(config)# policy-map parent
Switch(config-pmap)# class vlan101
Switch(config-pmap-c)# bandwidth percent 30
Switch(config-pmap-c)# service-policy policy1
Switch(config-pmap-c)# exit
Switch(config-pmap)# exit
Switch(config)# interface gigabitethernet1/1/1
Switch(config-if)# service-policy output parent
Configuring CBWFQ and IP Precedence-Based WRED
CBWFQ creates a queue for every class for which a class map is defined. Packets satisfying the match
criteria for a class accumulate in the queue reserved for the class until they are sent, which occurs when
the queue is serviced by the fair queue process. When the maximum packet threshold you defined for the
class is reached, any more packets destined for the class queue are dropped according to the tail drop or
the WRED mechanism.
WRED reduces the chances of tail drop by selectively dropping packets when the port 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.
You enable IP precedence-based WRED by using the random-detect precedence-based policy-map
class configuration command in an egress policy-map attached to an ES port. This command allows for
preferential drop treatment among packets with different IP precedence values. The WRED algorithm
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