Class-based policing allows you to control the maximum rate of traffic that is transmitted or received on an interface. Class-based policing is often configured on interfaces at the edge of a network to limit traffic into or out of the network.
Finding Feature Information
For the latest feature information and caveats, see the release notes for your platform and software release. To find information about the features documented in this module, and to see a list of the releases in which each feature is supported, see the"Feature Information for Class-Based Policing" section.
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•Information About Class-Based Policing
•Restrictions for Class-Based Policing
•How to Configure Class-Based Policing
•Configuration Examples for Class-Based Policing
•Feature Information for Class-Based Policing
Information About Class-Based Policing
Before configuring class-based policing, you should understand the following concepts:
•Class-Based Policing Functionality
•Benefits of Class-Based Policing
Class-Based Policing Functionality
The Class-Based Policing feature performs the following functions:
•Limits the input or output transmission rate of a class of traffic based on user-defined criteria.
•Marks packets by setting the ATM Cell Loss Priority (CLP) bit, Frame Relay Discard Eligibility (DE) bit, IP precedence value, IP differentiated services code point (DSCP) value, MPLS experimental value, and quality of service (QoS) group.
Class-based policing allows you to control the maximum rate of traffic transmitted or received on an interface. The Class-Based Policing feature is applied when you attach a traffic policy that contains the class-based policing configuration to an interface.
The Class-Based Policing feature works with a token bucket mechanism. There are currently two types of token bucket algorithms: a single token bucket algorithm and a two-token bucket algorithm. A single token bucket system is used when the violate-action option is not specified, and a two-token bucket system is used when the violate-action option is specified.
Benefits of Class-Based Policing
Bandwidth Management Through Rate Limiting
Class-based policing allows you to control the maximum rate of traffic transmitted or received on an interface. Class-based policing is often configured on interfaces at the edge of a network to limit traffic into or out of the network. In most class-based policing configurations, traffic that falls within the rate parameters is transmitted, whereas traffic that exceeds the parameters is dropped or transmitted with a different priority.
Packet marking allows you to partition your network into multiple priority levels or classes of service (CoS). A packet is marked and these markings can be used to identify and classify traffic for downstream devices.
•Use class-based policing to set the IP precedence or DSCP values for packets entering the network. Networking devices within your network can then use the adjusted IP precedence values to determine how the traffic should be treated.
•Use class-based policing to assign packets to a QoS group. The router uses the QoS group to determine how to prioritize packets.
Traffic can be marked without using the Class-Based Policing feature. If you want to mark traffic but do not want to use class-based policing, see the "Marking Network Traffic" module.
Restrictions for Class-Based Policing
Class-based policing can be configured on an interface or a subinterface, but it is not supported on EtherChannel or tunnel interfaces.
How to Configure Class-Based Policing
See the following sections for configuration tasks for the Class-Based Policing feature. Each task is identified as optional or required.
•Configuring Traffic Policing (required)
•Verifying Class-Based Traffic Policing (optional)
•Monitoring and Maintaining Traffic Policing (optional)
Configuring Traffic Policing
Router(config-pmap-c)# police bps burst-normal burst-max conform-action action exceed-action action violate-action action
Specifies a maximum bandwidth usage by a traffic class.
Verifying Class-Based Traffic Policing
Use the show policy-map interface command to verify that the Class-Based Policing feature is configured on your interface. If the feature is configured on your interface, the show policy-map interface command output displays policing statistics:
Router# show policy-map interface
1000000 bps, 10000 limit, 10000 extended limit
conformed 0 packets, 0 bytes; action: transmit
exceeded 0 packets, 0 bytes; action: drop
conformed 0 bps, exceed 0 bps, violate 0 bps
Check the interface type. Verify that class-based policing is supported on your interface. See the "Restrictions for Class-Based Policing" section.
Monitoring and Maintaining Traffic Policing
Router# show policy-map
Displays all configured policy maps.
Router# show policy-map policy-map-name
Displays the user-specified policy map.
Router# show policy-map interface
Displays statistics and configurations of all input and output policies that are attached to an interface.
Configuration Examples for Class-Based Policing
This section provides the following configuration example:
•Example: Configuring a Service Policy That Includes Traffic Policing
Example: Configuring a Service Policy That Includes Traffic Policing
In the following example, class-based policing is configured with the average rate at 8000 bits per second, the normal burst size at 1000 bytes, and the excess burst size at 1000 bytes for all packets leaving the interface.
police 8000 1000 1000 conform-action transmit exceed-action set-qos-transmit 1
service-policy output police-setting
The treatment of a series of packets leaving FastEthernet interface 1/1/1 depends on the size of the packet and the number of bytes remaining in the conform and exceed token buckets. The series of packets are policed based on the following rules:
•If the previous arrival of the packet was at T1 and the current arrival of the packet is at T, the bucket is updated with T - T1 worth of bits based on the token arrival rate. The refill tokens are placed in the conform bucket. If the tokens overflow the conform bucket, the overflow tokens are placed in the exceed bucket. The token arrival rate is calculated as follows:
(time between packets < which is equal to T - T1 > * policer rate)/8 bytes
•If the number of bytes in the conform bucket is greater than the length of the packet (for example, B), then the packet conforms and B bytes should be removed from the bucket. If the packet conforms, B bytes are removed from the conform bucket and the conform action is taken. The exceed bucket is unaffected in this scenario.
•If the number of bytes in the conform bucket is less than the length of the packet, but the number of bytes in the exceed bucket is greater than the length of the packet (for example, B), the packet exceeds and B bytes are removed from the bucket.
•If the number bytes in the exceed bucket B is fewer than 0, the packet violates the rate and the violate action is taken. The action is complete for the packet.
In this example, the initial token buckets starts full at 1000 bytes. If a 450-byte packet arrives, the packet conforms because enough bytes are available in the conform token bucket. The conform action (send) is taken by the packet, and 450 bytes are removed from the conform token bucket (leaving 550 bytes).
If the next packet arrives 0.25 seconds later, 250 bytes are added to the conform token bucket ((0.25 * 8000)/8), leaving 800 bytes in the conform token bucket. If the next packet is 900 bytes, the packet does not conform because only 800 bytes are available in the conform token bucket.
The exceed token bucket, which starts full at 1000 bytes (as specified by the excess burst size, is then checked for available bytes. Because enough bytes are available in the exceed token bucket, the exceed action (set the QoS transmit value of 1) is taken, and 900 bytes are taken from the exceed bucket (leaving 100 bytes in the exceed token bucket).
If the next packet arrives 0.40 seconds later, 400 bytes are added to the token buckets ((.40 * 8000)/8). Therefore, the conform token bucket now has 1000 bytes (the maximum number of tokens available in the conform bucket, and 200 bytes overflow the conform token bucket (because only 200 bytes were needed to fill the conform token bucket to capacity). These overflow bytes are placed in the exceed token bucket, giving the exceed token bucket 300 bytes.
If the arriving packet is 1000 bytes, the packet conforms because enough bytes are available in the conform token bucket. The conform action (transmit) is taken by the packet, and 1000 bytes are removed from the conform token bucket (leaving 0 bytes).
If the next packet arrives 0.20 seconds later, 200 bytes are added to the token bucket ((.20 * 8000)/8). Therefore, the conform bucket now has 200 bytes. If the arriving packet is 400 bytes, the packet does not conform because only 200 bytes are available in the conform bucket. Similarly, the packet does not exceed because only 300 bytes are available in the exceed bucket. Therefore, the packet violates and the violate action (drop) is taken.
Class-Based Quality of Service MIB
To locate and download MIBs for selected platforms, Cisco IOS XE Software releases, and feature sets, use Cisco MIB Locator found at the following URL:
A Single Rate Three Color Marker
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Feature Information for Class-Based Policing
Table 1 lists the release history for this feature.
Use Cisco Feature Navigator to find information about platform support and software image support. Cisco Feature Navigator enables you to determine which Cisco IOS XE Software images support a specific software release, feature set, or platform. To access Cisco Feature Navigator, go to http://www.cisco.com/go/cfn. An account on Cisco.com is not required.
Note Table 1 lists only the Cisco IOS XE Software release that introduced support for a given feature in a given Cisco IOS XE Software release train. Unless noted otherwise, subsequent releases of that Cisco IOS XE Software release train also support that feature.
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