27-7
Cisco ME 3800X and 3600X Switch Software Configuration Guide
OL-23400-01
Chapter 27 Configuring QoS
Understanding QoS
You can use the class map match-any class-map name global configuration command to define a
classification with any of the listed criteria.
In class-map configuration mode, you use the match command to specify the classification criteria. If a
packet matches the configured criteria, it belongs to a specific class and is forwarded according to the
specified policy. For example, you can use the match class-map command with CoS, IP DSCP, IP
precedence, or MPLS EXP values.You can also match an access group, a QoS group, or a VLAN ID or
inner VLAN ID or VLAN ID range for per-port, per-VLAN QoS.
For an input policy map, you cannot configure both an IP classification (match ip dscp, match ip
precedence, match ip acl) and a non-IP classification (match mac acl) in the same policy map or class
map.
This example shows how to create a class map example to define a class that matches any of the listed
criteria. In this example, if a packet is received with the DSCP equal to 32 or a 40, the pac ket is identified
(classified) by the class map.
Switch(config)#
class-map match-any example
Switch(config-cmap)#
match ip dscp 32
Switch(config-cmap)#
match ip dscp 40
Switch(config-cmap)#
exit
Classification Based on Layer 2 CoS
You use the match cos command to classify Layer 2 traffic based on the CoS value, which ranges from 0
to 7.
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 the 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 delivers 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
