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Understanding QoS
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.
The policy map can contain the police and police aggregate policy-map class configuration commands,
which define the policer, the bandwidth limitations of the traffic, and the action to take if the limits are
exceeded.
To make the policy map effective, you attach it to a port by using the service-policy interface
configuration command.
For more information, see the
information, see the
Policing and Marking
After a packet is classified and has a DSCP-based or CoS-based QoS label assigned to it, the policing
and marking process can begin as shown in
Policing involves creating a policer that specifies the bandwidth limits for the traffic. Packets that exceed
the limits are out of profile or nonconforming. Each policer decides on a packet-by-packet basis whether
the packet is in or out of profile and specifies the actions on the packet. These actions, carried out by the
marker, include passing through the packet without modification, dropping the packet, or modifying
(marking down) the assigned DSCP of the packet and allowing the packet to pass through. The
configurable policed-DSCP map provides the packet with a new DSCP-based QoS label. For information
on the policed-DSCP map, see the
the same queues as the original QoS label to prevent packets in a flow from getting out of order.
All traffic, regardless of whether it is bridged or routed, is subjected to a policer, if one is configured.
Note
As a result, bridged packets might be dropped or might have their DSCP or CoS fields modified when
they are policed and marked.
You can create these types of policers:
•
•
Note
The 10-Gigabit Ethernet interfaces do not support policing.
Catalyst 3750 Switch Software Configuration Guide
32-8
"Configuring a QoS Policy" section on page
Individual
QoS applies the bandwidth limits specified in the policer separately to each matched traffic class.
You configure this type of policer within a policy map by using the police policy-map class
configuration command.
Aggregate
QoS applies the bandwidth limits specified in an aggregate policer cumulatively to all matched
traffic flows. You configure this type of policer by specifying the aggregate policer name within a
policy map by using the police aggregate policy-map class configuration command. You specify the
bandwidth limits of the policer by using the mls qos aggregate-policer global configuration
command. In this way, the aggregate policer is shared by multiple classes of traffic within a policy
map.
"Policing and Marking" section on page
Figure
32-4.
"Mapping Tables" section on page
Chapter 32
Configuring QoS
32-8. For configuration
32-38.
32-10. Marked-down packets use
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