MQC Traffic Shaping Overhead Accounting for ATM

First Published: December 4, 2006
Last Updated: March 21, 2011

The MQC Traffic Shaping Overhead Accounting for ATM feature enables a broadband aggregation system (BRAS) to account for various encapsulation types when applying quality of service (QoS) functionality to packets. Typically, in Ethernet digital subscriber line (DSL) environments, the encapsulation from the router to the digital subscriber line access multiplexer (DSLAM) is Gigabit Ethernet and the encapsulation from the DSLAM to the customer premises equipment (CPE) is ATM. ATM overhead accounting enables the router to account for ATM encapsulation on the subscriber line and for the overhead added by cell segmentation. This functionality enables the service provider to prevent overruns at the subscriber line and ensures that the router executes QoS features on the actual bandwidth used by ATM packets.

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Contents

•Prerequisites for Traffic Shaping Overhead Accounting for ATM

•Restrictions for Traffic Shaping Overhead Accounting for ATM

•Information About Traffic Shaping Overhead Accounting for ATM

•How to Configure Traffic Shaping Overhead Accounting for ATM

•Configuration Examples for Traffic Shaping Overhead Accounting for ATM

•Additional References

•Command Reference

•Feature Information for MQC Traffic Shaping Overhead Accounting for ATM

Prerequisites for Traffic Shaping Overhead Accounting for ATM

Traffic classes must be configured using the class-map command.

Restrictions for Traffic Shaping Overhead Accounting for ATM

•The router supports ATM overhead accounting only for the shape and bandwidth commands.

•If you enable ATM overhead accounting on a child policy, then you must enable ATM overhead accounting on the parent policy.

•In a policy map, you must either enable ATM overhead accounting for all classes in the policy or disable overhead accounting for all classes in the policy. You cannot enable overhead accounting for some classes and disable overhead accounting for other classes in the same policy.

•The encapsulation type used within a policy map and between the parent policy map and the child policy map (in a hierarchical policy map structure) must be consistent.

•When you enter the show policy-map session command, the resulting classification byte counts and the queuing feature byte counts do not match. This is because the classification byte count does not consider overhead, whereas the queuing features do consider overhead.

Note This restriction applies to the Cisco 10000 series router only. This restriction does not apply to the Cisco 7600 series router.

•You must attach a policy map that is configured with ATM overhead accounting to only an Ethernet interface (or an IP session on an Ethernet interface).

Information About Traffic Shaping Overhead Accounting for ATM

Before configuring traffic shaping overhead accounting for ATM, you should understand the following concepts:

•Benefits of Traffic Shaping Overhead Accounting for ATM

•BRAS and Encapsulation Types

•Subscriber Line Encapsulation Types

•ATM Overhead Calculation

•ATM Overhead Accounting and Hierarchical Policies

Benefits of Traffic Shaping Overhead Accounting for ATM

The Traffic Shaping Overhead Accounting for ATM feature enables the broadband aggregation system (BRAS) to account for various encapsulation types when applying QoS to packets. Typically, in Ethernet digital subscriber line (DSL) environments, the encapsulation from the BRAS to the DSLAM is Gigabit Ethernet and the encapsulation from the DSLAM to the CPE is ATM. ATM overhead accounting enables the BRAS to account for ATM encapsulation on the subscriber line and for the overhead added by cell segmentation. This functionality enables the service provider to prevent overruns at the subscriber line and ensures that the router executes QoS features on the actual bandwidth used by ATM subscriber traffic.

BRAS and Encapsulation Types

Broadband aggregation system (BRAS) uses the encapsulation type that is configured for the DSLAM-CPE side to calculate the ATM overhead per packet.

DSLAM-CPE encapsulation types are based on Subnetwork Access Protocol (SNAP) and multiplexer (MUX) formats of ATM adaptation layer 5 (AAL5), followed by routed bridge (RBE), x-1483, x-dot1q-rbe, IP, PPP over Ethernet (PPPoE), or PPP over ATM (PPPoA) encapsulations. Because the DSLAM treats IP and PPPoE packets as payload, the BRAS does not account for IP and PPPoE encapsulations.

On the BRAS-DSLAM side, encapsulation is IEEE 802.1Q VLAN or Q-in-Q (qinq). However, because the DSLAM removes the BRAS-DSLAM encapsulation, the BRAS does not account for 802.1Q or qinq encapsulation.

AAL5 segmentation processing adds the additional overhead of the 5-byte cell headers, the AAL5 Common Part Convergence Sublayer (CPCS) padding, and the AAL5 trailer. For more information, see the "ATM Overhead Calculation" section.

Subscriber Line Encapsulation Types

The router supports the following subscriber line encapsulation types:

•snap-rbe

•mux-rbe

•snap-dot1q-rbe

•mux-dot1q-rbe

•snap-pppoa

•mux-pppoa

•snap-1483routed

•mux-1483routed

Note The encapsulation types listed above are for AAL5, qinq, and dot1q encapsulations. User-defined encapsulations with offsets based on the platform in use are also supported. (For the Cisco 10000 series router, valid offsets are -63 to +63. For the Cisco 7600 series router, valid offsets are -48 to +48.)

ATM Overhead Calculation

The Traffic Shaping Overhead Accounting for ATM feature prevents oversubscription of a subscriber line by accounting for the ATM encapsulation overhead at the BRAS. When calculating the ATM overhead, the Traffic Shaping Overhead Accounting for ATM feature considers the following:

•The encapsulation type used by the BRAS

•The CPCS trailer overhead

•The encapsulation type used between the DSLAM and the CPE

The offset size (a parameter used to calculate ATM overhead accounting) is calculated using the following formula:

Offset size in bytes = (CPCS trailer overhead) + (DSLAM to CPE) - (BRAS encapsulation type)

See Table 1 for the offset sizes, in bytes, derived from this formula.

This offset size, along with the packet size and packet assembler/disassembler (PAD) byte overhead in the CPCS, is used by the router to calculate the ATM overhead accounting rate.

Note A CPCS trailer overhead of 8 bytes corresponds to AAL5. A CPCS trailer overhead of 4 bytes corresponds to AAL3, but AAL3 is not supported.

Table 1 Offset Sizes, in Bytes, Used for ATM Overhead Calculation

Encapsulation Type in Use
BRAS
CPCS Trailer Overhead
DSLAM to CPE
Offset Size

dot1q mux-1483routed

18

8

3

-7

dot1q snap-1483routed

18

8

6

-4

dot1q mux-rbe

18

8

14

4

dot1q snap-rbe

18

8

24

14

dot1q mux-dot1q-rbe

18

8

18

8

dot1q snap-dot1q-rbe

18

8

28

18

qot1q mux-pppoa

18 + 6

8

2

-14

qot1q snap-pppoa

18 + 6

8

4

-12

qinq mux-1483routed

22

8

3

-11

qinq snap-1483routed

22

8

6

-8

qinq mux-rbe

22

8

14

0

qinq snap-rbe

22

8

24

10

qinq mux-dot1q-rbe

22

8

18

4

qing snap-dot1q-rbe

22

8

28

14

qinq mux-pppoa

22 + 6

8

2

-18

qinq snap-pppoa

22 + 6

8

4

-16

ATM Overhead Accounting and Hierarchical Policies

In hierarchical policies, you can enable ATM overhead accounting for shaping and bandwidth on parent policies and child policies. You are not required to enable ATM overhead accounting on a traffic class that does not contain the bandwidth or shape command. If you enable ATM overhead accounting on a child policy, then you must enable ATM overhead accounting on the parent policy. The parent and child classes must specify the same encapsulation type when ATM overhead accounting is enabled.

How to Configure Traffic Shaping Overhead Accounting for ATM

This section contains the following tasks.

•Configuring Traffic Shaping Overhead Accounting for ATM in a Hierarchical Policy (required)

•Verifying the Configuration of Traffic Shaping Overhead Accounting for ATM (optional)

Configuring Traffic Shaping Overhead Accounting for ATM in a Hierarchical Policy

To configure traffic shaping overhead accounting for ATM in a hierarchical policy map structure, perform the following steps.

SUMMARY STEPS

1. enable

2. configure terminal

3. policy-map policy-map-name

4. class class-map-name

5. bandwidth {bandwidth-kbps | percent percentage | remaining percent percentage} [account {qinq | dot1q} [aal5] {subscriber-encapsulation | user-defined offset}]

6. bandwidth remaining ratio ratio [account {qinq | dot1q} [aal5] {subscriber-encapsulation | user-defined offset}]

7. shape [average | peak] mean-rate [burst-size] [excess-burst-size] [account {qinq | dot1q} [aal5] {subscriber-encapsulation | user-defined offset}]

8. end

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 Business

Creates or modifies the child policy and enters policy-map configuration mode.

•Enter the policy map name. This is the name of the child policy and can be a maximum of 40 alphanumeric characters.

Step 4 

class class-map-name

Example:

Router(config-pmap)# class video

Assigns the traffic class that you specify for the policy map and enters policy-map class configuration mode.

•Enter the traffic class name. This is the name of the previously configured class map and can be a maximum of 40 alphanumeric characters.

Step 5 

bandwidth {bandwidth-kbps | percent percentage | remaining percent percentage} [account {qinq | dot1q} [aal5] {subscriber-encapsulation | user-defined offset}]

Example:

Router(config-pmap-c)# bandwidth 8000 account dot1q aal5 snap-pppoa

Enables Class-Based Weighted Fair Queuing (CBWFQ) on the basis of the keywords and arguments specified, such as the following:

•bandwidth-kbps—Specifies or modifies the minimum bandwidth allocated for a class that belongs to a policy map. Valid values are from 8 to 2488320, which represents from 1 to 99 percent of the link bandwidth.

•percent percentage—Specifies or modifies the minimum percentage of the link bandwidth allocated for a class that belongs to a policy map. Valid values are from 1 to 99.

•remaining percent percentage—Specifies or modifies the minimum percentage of unused link bandwidth allocated for a class that belongs to a policy map. Valid values are from 1 to 99.

•account—Enables ATM overhead accounting.

•qinq—Specifies queue-in-queue encapsulation as the BRAS-DSLAM encapsulation type.

•dot1q—Specifies IEEE 802.1Q VLAN encapsulation as the BRAS-DSLAM encapsulation type.

•aal5—Specifies the ATM adaptation layer 5 that supports connection-oriented variable bit rate (VBR) services.

•subscriber-encapsulation—Specifies the encapsulation type at the subscriber line. For more information, see the "Subscriber Line Encapsulation Types" section.

•user-defined—Specifies the offset size that the router uses when calculating the ATM overhead.

•offset—Specifies the offset size when calculating ATM overhead. Valid values are from -63 to +63 bytes.

Note For the Cisco 7600 series router, valid values are from -48 to +48 bytes.

Step 6 

bandwidth remaining ratio ratio [account {qinq | dot1q} [aal5] {subscriber-encapsulation | user-defined offset}]

Example:

Router(config-pmap-c)# bandwidth remaining ratio 10 account dot1q aal5 snap-pppo

(Optional) Specifies the bandwidth-remaining ratio for the subinterface along with ATM accounting parameters:

•ratio—Specifies the bandwidth-remaining ratio for the subinterface. Valid values are 1 to 100. The default value is 1.

Note For the Cisco 7600 series router, valid values are from 1 to 10000. The default value is 1.

•account—Enables ATM overhead accounting.

•qinq—Specifies queue-in-queue encapsulation as the BRAS-DSLAM encapsulation type.

•dot1q—Specifies IEEE 802.1Q VLAN encapsulation as the BRAS-DSLAM encapsulation type.

•aal5—Specifies the ATM adaptation layer 5 that supports connection-oriented VBR services.

•subscriber-encapsulation—Specifies the encapsulation type at the subscriber line. For more information, see the "Subscriber Line Encapsulation Types" section.

•user-defined—Specifies the offset size that the router uses when calculating the ATM overhead.

•offset—Specifies the offset size, in bytes, when calculating ATM overhead. Valid values are from -63 to +63.

Note For the Cisco 7600 series router, valid values are from -48 to +48.

Step 7 

shape [average | peak] mean-rate [burst-size] [excess-burst-size][account {qinq | dot1q} [aal5] {subscriber-encapsulation | user-defined offset}]

Example:

Router(config-pmap-c)# shape 8000 account qinq aal5 snap-dot1q-rbe

Shapes traffic to the indicated bit rate and enables ATM overhead accounting on the basis of the keywords and arguments specified, such as the following:

•average—(Optional) The committed burst (Bc) that specifies the maximum number of bits sent out in each interval.

•peak—(Optional) Specifies the maximum number of bits sent out in each interval (the Bc + excess burst [Be]). The Cisco 10000 router and the SIP400 (on the Cisco 7600 series router) do not support this option.

•mean-rate—Also called committed information rate (CIR). Indicates the bit rate used to shape the traffic, in bits per second.

•burst-size—(Optional) The number of bits in a measurement interval (Bc).

•excess-burst-size—(Optional) The acceptable number of bits permitted to go over the Be.

•account—Enables ATM overhead accounting.

•qinq—Specifies queue-in-queue encapsulation as the BRAS-DSLAM encapsulation type.

•dot1q—Specifies IEEE 802.1Q VLAN encapsulation as the BRAS-DSLAM encapsulation type.

•aal5—The ATM adaptation layer 5 that supports connection-oriented variable bit rate (VBR) services.

•subscriber-encapsulation—Specifies the encapsulation type at the subscriber line. For more information, see the "Subscriber Line Encapsulation Types" section.

•user-defined—Specifies the offset size that the router uses when calculating the ATM overhead.

•offset—Specifies the offset size when calculating ATM overhead. Valid values are from -63 to +63 bytes.

Note For the Cisco 7600 series router, valid values are from -48 to +48 bytes.

Step 8 

end

Example:

Router(config-pmap-c)# end

Exits policy-map class configuration mode.

Verifying the Configuration of Traffic Shaping Overhead Accounting for ATM

To verify the configuration of traffic shaping overhead accounting for ATM, perform the following steps.

SUMMARY STEPS

1. enable

2. show policy-map [policy-map-name]

3. show policy-map session

4. show running-config

5. exit

DETAILED STEPS

 
Command or Action
Purpose

Step 1 

enable

Example:

Router> enable

Enables privileged EXEC mode.

•Enter your password if prompted.

Step 2 

show policy-map [policy-map-name]

Example:

Router# show policy-map unit-test

(Optional) Displays the configuration of all classes for a specified policy map or of all classes for all existing policy maps.

•(Optional) Enter the policy map name. The name can be a maximum of 40 alphanumeric characters.

Step 3 

show policy-map session

Example:

Router# show policy-map session

(Optional) Displays the QoS policy map in effect for a IPoE/PPPoE session.

Step 4 

show running-config

Example:

Router# show running-config

(Optional) Displays the contents of the currently running configuration file.

Step 5 

exit

Example:

Router# exit

Exits privileged EXEC mode.

Configuration Examples for Traffic Shaping Overhead Accounting for ATM

This section provides the following configuration examples:

•Example: Enabling Traffic Shaping Overhead Accounting for ATM

•Example: Verifying Traffic Shaping Overhead Accounting for ATM

Example: Enabling Traffic Shaping Overhead Accounting for ATM

The following example shows how to enable ATM overhead accounting using a hierarchical policy map structure. The Child policy map has two classes: Business and Non-Business. The Business class has priority and is policed at 128,000 kbps. The Non-Business class has ATM overhead accounting enabled and has a bandwidth of 20 percent of the available bandwidth. The Parent policy map shapes the aggregate traffic to 256,000 kbps and enables ATM overhead accounting.

Notice that Layer 2 overhead accounting is not explicitly configured for the Business traffic class. If the class-default class of a parent policy has ATM overhead accounting enabled, you are not required to enable ATM overhead accounting on a child traffic class that does not contain the bandwidth or shape command. Therefore, in this example, the Business priority queue implicitly has ATM overhead accounting enabled because its parent class-default class has overhead accounting enabled. 

policy-map Child

 class Business

  priority

  police 128000 

 class Non-Business

  bandwidth percent 20 account dot1q aal5 snap-rbe-dot1q

  exit

 exit

policy-map Parent

 class class-default

  shape 256000 account dot1q aal5 snap-rbe-dot1q

  service-policy Child

In the following example, overhead accounting is enabled for bandwidth on the gaming and class-default class of the child policy map named subscriber_classes and on the class-default class of the parent policy map named subscriber_line. The voip and video classes do not have accounting explicitly enabled; these classes have ATM overhead accounting implicitly enabled because the parent policy has overhead accounting enabled. Notice that the features in the parent and child policies use the same encapsulation type. 

policy-map subscriber_classes

 class voip

  priority level 1

  police 8000

 class video

  priority level 2

  police 8000

 class gaming

  bandwidth remaining percent 80 account dot1q aal5 snap-rbe-dot1q

 class class-default

  bandwidth remaining percent 20 account dot1q aal5 snap-rbe-dot1q

policy-map subscriber_line

 class class-default

  bandwidth remaining ratio 10 account dot1q aal5 snap-rbe-dot1q

  shape average 512 account aal5 dot1q snap-rbe-dot1q

  service policy subscriber_classes

Example: Verifying Traffic Shaping Overhead Accounting for ATM

The following output from the show policy-map interface command indicates that ATM overhead accounting is enabled for shaping and disabled for bandwidth: 

Router# show policy-map interface

Service-policy output:unit-test


Class-map: class-default (match-any)

100 packets, 1000 bytes

30 second offered rate 800 bps, drop rate 0 bps

Match: any

shape (average) cir 154400, bc 7720, be 7720

target shape rate 154400

overhead accounting: enabled

bandwidth 30% (463 kbps)

overhead accounting: disabled


queue limit 64 packets

(queue depth/total drops/no-buffer drops) 0/0/0

(packets output/bytes output) 100/1000

The following output from the show policy-map session command indicates that ATM overhead accounting is enabled for shaping. 

Router# show policy-map session output 


SSS session identifier 2 -


Service-policy output:  ATM_OH_POLICY 



    Class-map: class-default (match-any)

      0 packets, 0 bytes

      30 second offered rate 0 bps, drop rate 0 bps

      Match: any 

      Queueing

      queue limit 2500 packets

      (queue depth/total drops/no-buffer drops) 0/0/0

      (pkts output/bytes output) 0/0

      shape (average) cir 10000000, bc 40000, be 40000

      target shape rate 10000000

       Overhead Accounting Enabled

The following output from the show running-config command indicates that ATM overhead accounting is enabled for shaping. The BRAS-DSLAM encapsulation is dot1q and the subscriber line encapsulation is snap-rbe based on the AAL5 service. 

subscriber policy recording rules limit 64

no mpls traffic-eng auto-bw timers frequency 0

call rsvp-sync

!

controller T1 2/0

framing sf

linecode ami

!

controller T1 2/1

framing sf

linecode ami

!

!

policy-map unit-test

class class-default

shape average percent 10 account dot1q aal5 snap-rbe

!

Additional References

The following sections provide references related to traffic shaping overhead accounting for ATM.

Related Documents

Related Topic
Document Title

QoS commands: complete command syntax, command modes, command history, defaults, usage guidelines, and examples

Cisco IOS Quality of Service Solutions Command Reference

Modular Quality of Service (QoS) Command-Line Interface (CLI) (MQC), hierarchical policies, policy maps

"Applying QoS Features Using the MQC" module

Policing and shaping traffic

"Policing and Shaping Overview" module


Standards

Standard
Title

No new or modified standards are supported by this feature, and support for existing standards has not been modified by this feature.

—


MIBs

MIB
MIBs Link

No new or modified MIBs are supported by this feature, and support for existing MIBs has not been modified by this feature.

To locate and download MIBs for selected platforms, Cisco IOS releases, and feature sets, use Cisco MIB Locator found at the following URL:

http://www.cisco.com/go/mibs


RFCs

RFC
Title

No new or modified RFCs are supported by this feature, and support for existing RFCs has not been modified by this feature.

—


Technical Assistance

Description
Link

The Cisco Support and Documentation website provides online resources to download documentation, software, and tools. Use these resources to install and configure the software and to troubleshoot and resolve technical issues with Cisco products and technologies. Access to most tools on the Cisco Support and Documentation website requires a Cisco.com user ID and password.

http://www.cisco.com/cisco/web/support/index.html


Command Reference

The following commands are introduced or modified in the feature or features documented in this module. For information about these commands, see the Cisco IOS Quality of Service Solutions Command Reference at http://www.cisco.com/en/US/docs/ios/qos/command/reference/qos_book.html. For information about all Cisco IOS commands, use the Command Lookup Tool at http://tools.cisco.com/Support/CLILookup or a Cisco IOS master commands list.

•bandwidth (policy-map class)

•bandwidth remaining ratio

•shape (policy-map class)

•show policy-map interface

•show policy-map session

•show running-config

Feature Information for MQC Traffic Shaping Overhead Accounting for ATM

Table 2 lists the release history for this feature.

Not all commands may be available in your Cisco IOS software release. For release information about a specific command, see the command reference documentation.

Use Cisco Feature Navigator to find information about platform support and software image support. Cisco Feature Navigator enables you to determine which Cisco IOS and Catalyst OS 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 2 lists only the Cisco IOS software release that introduced support for a given feature in a given Cisco IOS software release train. Unless noted otherwise, subsequent releases of that Cisco IOS software release train also support that feature.

Table 2 Feature Information for MQC Traffic Shaping Overhead Accounting for ATM 

Feature Name
Releases
Feature Information

MQC Traffic Shaping Overhead Accounting for ATM

12.2(31)SB2
12.2(33)SRC
12.2(33)SB

The MQC Traffic Shaping Overhead Accounting for ATM feature enables a broadband aggregation system (BRAS) to account for various encapsulation types when applying QoS functionality to packets.

In Release 12.2(31)SB2, this feature was introduced and implemented on the Cisco 10000 series router for the PRE3.

In Release 12.2(33)SRC, support was added for the Cisco 7600 series router.

In Release 12.2(33)SB, support was added for the Cisco 7300 series router.

The following commands were introduced or modified: bandwidth (policy-map class), bandwidth remaining ratio, shape (policy-map class), show policy-map interface, show policy-map session, show running-config.


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Any Internet Protocol (IP) addresses and phone numbers used in this document are not intended to be actual addresses and phone numbers. Any examples, command display output, network topology diagrams, and other figures included in the document are shown for illustrative purposes only. Any use of actual IP addresses or phone numbers in illustrative content is unintentional and coincidental.

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