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Understanding QoS
Step 3
Attach the traffic policy to an interface.
You use the service-policy interface configuration command to attach the policy map to an interface for
packets entering or leaving the interface. You must specify whether the traffic policy characteristics
should be applied to incoming or outgoing packets. For example, entering the service-policy output
class1 interface configuration command attaches all the characteristics of the traffic policy named class1
to the specified interface. All packets leaving the specified interface are evaluated according to the
criteria specified in the traffic policy named class1.
Input and Output Policies
Policy maps are either input policy maps or output policy maps, attached to packets as they enter or leave
the switch by service policies applied to interfaces. Input policy maps usually perform policing and
marking on received traffic. Policed packets can be dropped or reduced in priority (marked down) if they
exceed the maximum permitted rates. Output policy maps perform scheduling and queuing on traffic as
it leaves the switch.
Input policies and output policies have the same basic structure; the difference is in the characteristics
they regulate.
The maximum number of policy maps that you can configure is 256. You can apply one input policy map
and one output policy map to an interface.
Figure 30-2
Apply
input
policy
Apply
output
policy
Input Policy Maps
Input policy map classification criteria include matching a CoS, a DSCP, or an IP precedence value or
matching an ACL. Input policy maps can have any of these actions:
•
•
•
Only input policies provide matching on access groups, and only output policies provide matching on
QoS groups. You can assign a QoS group number in an input policy, and match it in the output policy.
The class class-default is used in a policy map for any traffic that does not explicitly match any other
class in the policy map. Input policy maps do not support queuing and scheduling commands, such as
bandwidth, queue-limit, priority, and shape average.
You can configure a maximum of 32 total classes in an input policy.
Cisco ME 3400 Ethernet Access Switch Software Configuration Guide
30-4
Figure 30-2
shows the relationship of input and output policies.
Input and Output Policy Relationship
Ingress
port
Setting or marking a CoS, a DSCP, an IP precedence, or a QoS group value
Individual policing
Aggregate policing
Switching
Egress
and
forwarding
Chapter 30
Configuring QoS
Apply
output
policy
port
Apply
input
policy
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