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Software Configuration Guide—Release 12.2(25)SG
OL-7659-03
Chapter27 Configuring Quality of Service
Overview of QoS
You create a class map by using the class-map global configuration command. When you enter the
class-map command, the switch enters the class-map configuration mode. In this mode, you define the
match criteria for the traffic by using the match class-map configuration command.
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 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 can also 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 “Policing and
Marking” section on page 27-10.
A policy map also has these characteristics:
•A policy map can contain up to 255 class statements.
•You can have different classes within a policy map.
•A policy-map trust state supersedes an interface trust state.
For configuration information, see the “Configuring a QoS Policy” section on page 27-29.
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 Figure27-4.
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 pac ket 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 27-14.
You can create these types of policers:
•Individual
QoS applies the bandwidth limits specified in the policer separately to each matched traffic class for
each port/VLAN to which the policy map is attached to. You configure this type of policer within a
policy map by using the police command under policy-map class configuration mode.
•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 qos aggregate-policer global configuration command.
In this way, the aggregate policer is shared by multiple classes of traffic within a policy map.
•Flow or Microflow
With flow-based policing, all the identified flows are policed to the specified rate individually.
Because the flows are dynamic, key distinguishing fields must be configured in class maps. Two
flow-matching options are provided: source ip based (each flow with unique source IP address is
treated as a new flow) and destination ip based (each flow with unique destination IP address is
treated as new flow). For information on flow-based policer configuration, see “Configuring User
Based Rate Limiting” on page 36.