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.

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.

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

• What to Do Next
  

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