Regulating Packet Flow on a Per-Class Basis Using Class-Based Traffic Shaping

Class-based traffic shaping is a traffic-shaping mechanism (also known as a "traffic shaper"). A traffic shaper typically delays excess traffic using a buffer, or queueing mechanism, to hold packets and shape the flow when the data rate of the source is higher than expected. It holds and shapes traffic to a particular bit rate by using the token bucket mechanism.

Class-based traffic shaping is the Cisco-recommended traffic-shaping mechanism.

Using class-based traffic shaping, you can perform the following tasks:

• Configure traffic shaping on a per-traffic-class basis. It allows you to fine-tune traffic shaping for one or more classes, and it allows you to configure traffic shaping on a more granular level.
• Specify average rate or peak rate traffic shaping. Specifying peak rate shaping allows you to make better use of available bandwidth by allowing more data than the configured traffic shaping rate to be sent if the bandwidth is available.
• Configure traffic shaping in a hierarchical policy map structure. That is, traffic shaping is configured in a primary-level (parent) policy map, and other QoS features (for instance, CBWFQ and traffic policing) can be configured in the secondary-level (child) policy maps.

All of the benefits associated with traffic shaping also apply to Class-Based Traffic Shaping, but on a more granular level. For information about the benefits of traffic shaping, see the "Regulating Packet Flow Using Traffic Shaping" module.

With the class-based traffic shaping mechanism, traffic shaping can be configured in a hierarchical policy map structure; that is, traffic shaping is enabled in a primary-level (parent) policy map and other QoS features used with traffic shaping, such as class-based WFQ (CBWFQ) and traffic policing, can be enabled in a secondary-level (child) policy map. Traffic shaping is enabled by using the shape command (and specifying a rate) in a policy map.

CBWFQ allows you to fine-tune the way traffic is placed in a queue. For instance, you can specify that all voice traffic be placed in a high-priority queue and all traffic from a specific class be placed in a lower-priority queue.

If you want to use CBWFQ with the class-based traffic shaping mechanism a secondary-level (child) policy map must be created. This secondary-level (child) policy map is then used to configure CBWFQ by enabling the bandwidth command.

The following sample configuration illustrates how the class-based traffic shaping mechanism is configured in a hierarchical policy map structure:

enable
configure terminal
 policy-map policy_parent        ! This is the primary-level policy map.
  class class-default
   shape average 1000000         ! This enables traffic shaping.
   service-policy policy_child   ! This associates the policy maps.

With this configuration, WFQ is used as the default queueing mechanism for placing all the traffic in a queue.

In the following secondary-level (child) policy map, the alternative queueing mechanism CBWFQ is configured:

enable
configure terminal
 policy-map policy_child     ! This is the secondary-level policy map.
  class class-default
   bandwidth percent 50      ! This enables CBWFQ.

SUMMARY STEPS

1.    enable

2.    configure terminal

3.    policy-map policy-map-name

4.    class {class-name | class-default}

5.    shape [average | peak] mean-rate [burst-size] [excess-burst-size]

6.    service-policy policy-map-name

7.    end

8.    show policy-map

9.    show policy-map interface type number

10.    exit

DETAILED STEPS

	Command or Action	Purpose
Step 1	enable Example: Router> enable	Enables privileged EXEC mode. Enter your password if prompted.
Step 2	configure terminal Example: Router# configure terminal	Enters global configuration mode.
Step 3	policy-map policy-map-name Example: Router(config)# policy-map policy_parent	Specifies the name of the policy map created earlier and enters policy-map configuration mode. See the "Prerequisites" section on page 3 for more information. Enter the policy map name.
Step 4	class {class-name | class-default} Example: Router(config-pmap)# class class-default	Specifies the name of the class whose policy you want to create and enters policy-map class configuration mode. Enter the name of the class or enter the class-defaultkeyword.
Step 5	shape [average | peak] mean-rate [burst-size] [excess-burst-size] Example: Router(config-pmap-c)# shape average 1000000	Shapes traffic according to the keyword and rate specified. Enter the keyword and rate.
Step 6	service-policy policy-map-name Example: Router(config-pmap-c)# service-policy policy_child	Uses a service policy as a QoS policy within a policy map (called a hierarchical service policy). Enter the policy map name.
Step 7	end Example: Router(config-pmap-c)# end	Returns to privileged EXEC mode.
Step 8	show policy-map Example: Router# show policy-map	(Optional) Displays all configured policy maps.
Step 9	show policy-map interface type number Example: Router# show policy-map interface serial4/0/0	(Optional) Displays the packet statistics of all classes that are configured for all service policies either on the specified interface or subinterface or on a specific PVC on the interface. Enter the interface type and number.
Step 10	exit Example: Router# exit	(Optional) Exits privileged EXEC mode.

• What to Do Next
  

SUMMARY STEPS

1.    enable

2.    configure terminal

3.    policy-map policy-map-name

4.    class {class-name | class-default}

5.    bandwidth {bandwidth-kbps | remaining percent percentage | percent percentage}

6.    end

7.    show policy-map

8.    show policy-map interface type number

9.    exit

DETAILED STEPS

	Command or Action	Purpose
Step 1	enable Example: Router> enable	Enables privileged EXEC mode. Enter your password if prompted.
Step 2	configure terminal Example: Router# configure terminal	Enters global configuration mode.
Step 3	policy-map policy-map-name Example: Router(config)# policy-map policy1	Specifies the name of the policy map created earlier and enters policy-map configuration mode. See the "Prerequisites" section on page 3 for more information. Enter the policy map name.
Step 4	class {class-name | class-default} Example: Router(config-pmap)# class class-default	Specifies the name of the class whose policy you want to create and enters policy-map class configuration mode. Enter the name of the class or enter the class-default keyword.
Step 5	bandwidth {bandwidth-kbps | remaining percent percentage | percent percentage} Example: Router(config-pmap-c)# bandwidth percent 50 Example:	Specifies or modifies the bandwidth allocated for a class belonging to a policy map. Enter the amount of bandwidth as a number of kbps, a relative percentage of bandwidth, or an absolute amount of bandwidth. Note    The bandwidth command used here is only an example of a QoS feature than can be configured. The bandwidth command configures CBWFQ. You could also use the police command to configure traffic policing.
Step 6	end Example: Router(config-pmap-c)# end	Returns to privileged EXEC mode.
Step 7	show policy-map Example: Router# show policy-map	(Optional) Displays all configured policy maps.
Step 8	show policy-map interface type number Example: Router# show policy-map interface serial4/0/0	(Optional) Displays the packet statistics of all classes that are configured for all service policies either on the specified interface or subinterface or on a specific PVC on the interface. Enter the interface type and number.
Step 9	exit Example: Router# exit	(Optional) Exits privileged EXEC mode.