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Configuring QoS
Understanding QoS
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)
cs6 Match packets with CS6(precedence 6) dscp (110000)
cs7 Match packets with CS7(precedence 7) dscp (111000)
default Match packets with default dscp (000000)
ef Match packets with EF dscp (101110)
For more information on DSCP prioritization, see RFC-2597 (AF per-hop behavior), RFC-2598 (EF), or RFC-2475
(DSCP).
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