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Catalyst 3550 Multilayer Switch Software Configuration Guide
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Chapter 29 Configuring QoS
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. To make the policy map effective, you attach it to an interface
by using the service-policy interface configuration command.
The policy map also can contain commands that define the policer, the bandwidth limitations of the
traffic, and the action to take if the limits are exceeded. For more information, see the
A policy map has these characteristics:
•
A policy map can contain multiple class statements.
•
A separate policy-map class can exist for each type of traffic received through an interface.
•
The policy-map trust state and an interface trust state are mutually exclusive, and whichever is
configured last takes affect.
For configuration information, see the
“Configuring a QoS Policy” section on page 29-35
Policing and Marking
After a packet is classified and has an internal DSCP value 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 specifies the action to take for packets that
are in or out of profile. These actions, carried out by the marker, include passing through the packet
without modification, dropping the packet, or marking down the packet with a new DSCP value that is
obtained from the configurable policed-DSCP map. For information on the policed-DSCP map, see the
“Mapping Tables” section on page 29-10
You can create these types of policers:
•
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 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 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 uses a token bucket algorithm. As each frame is received by the switch, a token is added to the
bucket. The bucket has a hole in it and leaks at a rate that you specify as the average traffic rate in bits
per second. Each time a token is added to the bucket, the switch performs a check to determine if there
is enough room in the bucket. If there is not enough room, the packet is marked as nonconforming, and
the specified policer action is taken (dropped or marked down).