Cisco Catalyst 3560-X Software Configuration Manual page 922

Understanding QoS
In the QoS context, the permit and deny actions in the access control entries (ACEs) have different
meanings than with security ACLs:
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When creating an access list, remember that, by default, the end of the access list contains an implicit
Note
deny statement for everything if it did not find a match before reaching the end.
After a traffic class has been defined with the ACL, you can attach a policy to it. A policy might contain
multiple classes with actions specified for each one of them. A policy might include commands to
classify the class as a particular aggregate (for example, assign a DSCP) or rate-limit the class. This
policy is then attached to a particular port on which it becomes effective.
You implement IP ACLs to classify IP traffic by using the access-list global configuration command;
you implement Layer 2 MAC ACLs to classify non-IP traffic by using the mac access-list extended
global configuration command. For configuration information, see the
section on page
Classification Based on Class Maps and Policy Maps
A class map is a mechanism that you use to name a specific traffic flow (or class) and isolate it from all
other traffic. The class map defines the criteria used to match against a specific traffic flow to further
classify it. The criteria can include matching the access group defined by the ACL or matching a specific
list of DSCP or IP precedence values. If you have more than one type of traffic that you want to classify,
you can create another class map and use a different name. After a packet is matched against the
class-map criteria, you further classify it through the use of a policy map.
A policy map specifies which traffic class to act on. Actions can include trusting the CoS, DSCP, or IP
precedence values in the traffic class; setting a specific DSCP or IP precedence value in the traffic class;
or specifying the traffic bandwidth limitations and the action to take when the traffic is out of profile.
Before a policy map can be effective, you must attach it to a port.
You create a class map by using the class-map global configuration command or the class policy-map
configuration command. You should use the class-map command when the map is shared among many
ports. When you enter the class-map command, the switch enters the class-map configuration mode. In
this mode, you define the match criterion for the traffic by using the match class-map configuration
command.
You can configure a default class by using the class class-default policy-map configuration command.
Unclassified traffic (traffic that does not meet the match criteria specified in the traffic classes) is treated
as default traffic.
You create and name a policy map by using the policy-map global configuration command. When you
enter this command, the switch enters the policy-map configuration mode. In this mode, you specify the
actions to take on a specific traffic class by using the class, trust, or set policy-map configuration and
policy-map class configuration commands.
Catalyst 3750-X and 3560-X Switch Software Configuration Guide
1-8
If a match with a permit action is encountered (first-match principle), the specified QoS-related
action is taken.
If a match with a deny action is encountered, the ACL being processed is skipped, and the next ACL
is processed.
If no match with a permit action is encountered and all the ACEs have been examined, no QoS
processing occurs on the packet, and the switch offers best-effort service to the packet.
If multiple ACLs are configured on a port, the lookup stops after the packet matches the first ACL
with a permit action, and QoS processing begins.
1-49.
Chapter 1 Configuring QoS
"Configuring a QoS Policy"
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