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Cisco ME 3400 Ethernet Access Switch Software Configuration Guide
OL-9639-06
Chapter 33 Configuring QoS
Understanding QoS
This example shows how to create a class map to match a CoS value of 5:
Switch(config)# class-map premium
Switch(config-cmap)# match cos 5
Switch(config-cmap)# exit
Classification Based on IP Precedence
You can classify IPv4 traffic based on the packet IP precedence values, which range from 0 to 7.
This example shows how to create a class map to match an IP precedence value of 4:
Switch(config)# class-map sample
Switch(config-cmap)# match ip precedence 4
Switch(config-cmap)# exit
Classification Based on IP DSCP
When you classify IPv4 traffic based on IP DSCP value, and enter the match ip dscp class-map
configuration command, you have several classification options:
• Entering a specific DSCP value (0 to 63).
• Using the Default service, which corresponds to an IP precedence and DSCP value of 0. The default
per-hop behavior (PHB) is usually best-effort service.
• Using Assured Forwarding (AF) by entering the binary representation of the DSCP value. AF sets
the relative probability that a specific class of packets is forwarded when congestion occurs and the
traffic does not exceed the maximum permitted rate. AF per-hop behavior provides delivery of IP
packets in four different AF classes: AF11-13 (the highest), AF21-23, AF31-33, and AF41-43 (the
lowest). Each AF class could be allocated a specific amount of buffer space and drop probabilities,
specified by the binary form of the DSCP number. When congestion occurs, the drop precedence of
a packet determines the relative importance of the packet within the class. An AF41 provides the
best probability of a packet being forwarded from one end of the network to the other.
• Entering Class Selector (CS) service values of 1 to 7, corresponding to IP precedence bits in the ToS
field of the packet.
• Using Expedited Forwarding (EF) to specify a low-latency path. This corresponds to a DSCP value
of 46. EF services use priority queuing to preempt lower priority traffic classes.
This display shows the available classification options:
Switch(config-cmap)# match ip dscp ?
<0-63> Differentiated services codepoint value
af11 Match packets with AF11 dscp (001010)
af12 Match packets with AF12 dscp (001100)
af13 Match packets with AF13 dscp (001110)
af21 Match packets with AF21 dscp (010010)
af22 Match packets with AF22 dscp (010100)
af23 Match packets with AF23 dscp (010110)
af31 Match packets with AF31 dscp (011010)
af32 Match packets with AF32 dscp (011100)
af33 Match packets with AF33 dscp (011110)
af41 Match packets with AF41 dscp (100010)
af42 Match packets with AF42 dscp (100100)
af43 Match packets with AF43 dscp (100110)
cs1 Match packets with CS1(precedence 1) dscp (001000)
cs2 Match packets with CS2(precedence 2) dscp (010000)
cs3 Match packets with CS3(precedence 3) dscp (011000)
cs4 Match packets with CS4(precedence 4) dscp (100000)
cs5 Match packets with CS5(precedence 5) dscp (101000)