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QoS: L2 Overhead Specification for Shaping and Policing Parameters for Ethernet

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QoS: L2 Overhead Specification for Shaping and Policing Parameters for Ethernet

Table Of Contents

QoS: L2 Overhead Specification for Shaping and Policing Parameters for Ethernet

Contents

Configuring QoS: L2 Overhead Specification for Shaping and Policing Parameters for Ethernet

Restrictions

SUMMARY STEPS

DETAILED STEPS

Additional References

Related Documents

Technical Assistance

Command Reference

hw-module slot qos account layer2 encapsulation


QoS: L2 Overhead Specification for Shaping and Policing Parameters for Ethernet


The Layer 2 (L2) Overhead Specification for Shaping and Policing Parameters for Ethernet allows you to configure shaping and policing bit rates that count Layer 2 encapsulation frame lengths in addition to Layer 3 packet lengths.

Feature History for QoS: L2 Overhead Specification for Shaping and Policing Parameters for Ethernet

Release
Modification

12.0(28)S

This feature was introduced for the Cisco 12000 Series 4-Port Gigabit Ethernet ISE Line Card.


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Contents

This document describes the L2 Overhead Specification feature in Cisco IOS 12.(28)S and includes the following sections:

Configuring QoS: L2 Overhead Specification for Shaping and Policing Parameters for Ethernet

Additional References

Command Reference

Configuring QoS: L2 Overhead Specification for Shaping and Policing Parameters for Ethernet

This feature provides you with the option to configure shaping and policing QoS features that take the Layer 2 encapsulation overhead into account. Previously, outbound shaping and policing algorithms were based only on the Layer 3 packet length.

This feature is enabled with the hw-module slot slot-number qos account layer2 encapsulation {arpa | dot1q | length} global configuration command. When enabled, the feature is enabled on all ports of the line card in both ingress and egress directions. Layer 3 accounting is the default. The no form of the command enables the default. The show policy-map interface and show interface rate commands display the additional Layer 2 overhead length.

For the Cisco 12000 Series 4-Port Gigabit Ethernet ISE Line Card, the Layer 2 encapsulation overhead options to specify are as follows:

Keyword Option
Description

arpa

Specifies Ethernet frames with a header size of 14 bytes.

dot1q

Specifies 802.1Q Ethernet frames with a header size of 18 bytes.

length

Specifies a length in bytes of the Layer 2 encapsulation frame.


The specified byte length in the CLI is used only in the egress (Tx) direction. Output shaping or policing must be configured on an interface. The bit rate transmitted from the line card is determined by adding the Layer 3 length of the packet to the length defined in the CLI.

Incoming packets are shaped or policed based on the true Layer 2 length of the packets. For example, if the user-specified length is 14 bytes, and all packets egressing from any port are ARPA main interface packets, then the results are accurate. If some egress packets have VLAN encapsulations and some are non-VLAN (main interface) packets, then the results cannot be completely accurate because the encapsulation overheads are different. On the input side, Layer 2 encapsulation overhead is 14 bytes for ARPA main interface packets and 18 bytes for 802.1Q packets, regardless of the user-specified length.

Restrictions

For Any Transport over ATM (AToM) the user-specified byte length on the egress of the disposition router is still added to the length of the packet, though there is no Layer 2 transport encapsulation on the packet.

IPv6 does not currently support input/output Committed Access Rate (CAR).

SUMMARY STEPS

1. enable

2. configure terminal

3. hw-module slot slot-number qos account layer2 encapsulation {arpa | dot1q | length}

4. 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 

hw-module slot slot-number qos account layer2 encapsulation {arpa | dot1q | length}

Example:

Router(config)# hw-module slot 1 qos account layer2 encapsulation length 14

Enables Layer 2 overhead accounting for calculating policed or shaped bit rates.

In this example, a Layer 2 overhead of 14 bytes per frame is added to the egress traffic for interfaces on the line card in slot 1.

Step 4 

exit

Example:

Router(config)# exit

Exits global configuration mode.

Additional References

The following sections provide references related to QoS: L2 Overhead Specification for Shaping and Policing Parameters for Ethernet.

Related Documents

Related Topic
Document Title

Quality of service (QoS) commands: complete command syntax, command modes, command history, defaults, usage guidelines, and examples

Cisco IOS Quality of Service Solutions Command Reference, Release 12.3 T

Modular QoS Command-Line Interface (CLI) (MQC)

Cisco IOS Quality of Service Solutions Configuration Guide

Information about attaching policy maps to interfaces

Cisco IOS Quality of Service Solutions Configuration Guide

Traffic policing

Cisco IOS Quality of Service Solutions Configuration Guide


Technical Assistance

Description
Link

Technical Assistance Center (TAC) home page, containing 30,000 pages of searchable technical content, including links to products, technologies, solutions, technical tips, and tools. Registered Cisco.com users can log in from this page to access even more content.

http://www.cisco.com/public/support/tac/home.shtml


Command Reference

This section documents modified commands. All other commands used with this feature are documented in the Cisco IOS Release 12.0 command reference publications.

hw-module slot qos account layer2 encapsulation

To enable Layer 2 accounting for QoS, use the command in global configuration mode. To disable, use the no form of this command.

hw-module slot slot-number qos account layer2 encapsulation {arpa | dot1q | length}

no hw-module slot slot-number qos account layer2 encapsulation {arpa | dot1q | length}

Syntax Description

slot-number

Specifies the slot number of the line card to be configured for L2 overhead specification.

arpa

Specifies L2 overhead frames as Ethernet frames with a header size of
14 bytes.

dot1q

Specifies L2 overhead frames as 802.1Q Ethernet frames with a header size of 18 bytes.

length

Specifies length in bytes of the L2 overhead to be added for each egress frame.


Defaults

No Layer 2 encapsulation accounting is the default.

Command Modes

Global configuration

Command History

Release
Modification

12.0(28)S

This command was introduced.


Usage Guidelines

The no version of the command enables the default method of calculating the bit rate of traffic over an interface based on Layer 3 packet lengths.

The length parameter specifies Layer 2 overhead for output traffic only. The packets transmitted from the line card are shaped or policed by adding the user-specified length to the Layer 3 length of the output packets. The incoming packets are shaped or policed according to the actual Layer 2 lengths of the encapsulations, regardless of the user-specified length.

For Any Transport over ATM (AToM), the user specified byte length on the egress of the disposition router is still added to the length of the packet though there is no Layer 2 transport encapsulation on the packet.

The following example specifies that 16 bytes of Layer 2 encapsulation overhead are added for each Layer 3 packet egressing from the linecard:

Router(config)# hw-module slot 1 qos account layer2 encapsulation 16 

Related Commands

Command
Description

show interfaces rate-limit

Displays information about the committed access rate (CAR) for an interface

show policy-map interface

Displays the configuration of all classes configured for all service policies on the specified interface.