Ethernet Overhead Accounting

Last Updated: July 13, 2012

The Ethernet Overhead Accounting feature enables the router to account for downstream Ethernet frame headers when applying shaping to packets.

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Restrictions for Ethernet Overhead Accounting

  • Ethernet overhead accounting allows the automatic inclusion of downstream Ethernet frame headers in the shaped rate. However, policing is not supported for Ethernet overhead accounting
  • The router supports overhead accounting only for the shape and bandwidth commands.
  • If you enable overhead accounting on a child policy, then you must enable overhead accounting on the parent policy.
  • In a policy map, you must either enable 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.
  • When you enter the show policy-map interface command, the resulting classification byte counts and the queueing feature byte counts do not match. This mismatch occurs because the classification byte count does not consider overhead, whereas the queueing features do consider overhead.
  • You can enable overhead accounting for shaping and bandwidth on top-level parent policies, middle-level child policies, and bottom-level child policies.
  • If you enable overhead accounting on a parent policy, you are required to enable accounting on a child policy that is configured with the shape or bandwidth command. You are not required to enable accounting on a child policy that does not have the shape or bandwidth command configured.

Information About Ethernet Overhead Accounting

Benefits of Ethernet Overhead Accounting

The Ethernet Overhead Accounting feature enables the router to account for downstream Ethernet frame headers when applying shaping to packets. A user-defined offset specifies the number of overhead bytes that the router is to use when calculating the overhead per packet Valid offset values are from +63 bytes to -63 bytes of overhead. Before applying shaping, the router calculates teh overhead.

Ethernet interfaces and subinterfaces support overhead accounting. Using the shape or bandwidth command, you can configure accounting per VLAN and per port.

Subscriber Line Encapsulation Types

The subscriber-encap option of the shape and bandwidth commands specifies the encapsulation type at the subscriber line. The router supports the following subscriber line encapsulation types:

  • snap-1483routed
  • mux-1483routed
  • snap-dot1q-rbe
  • mux-dot1q-rbe
  • snap-pppoa
  • mux-pppoa
  • snap-rbe
  • mux-rbe

Overhead Calculation on the Router

When calculating overhead for traffic shaping, the router considers the encapsulation type used between the BRAS and the DSLAM and between the DSLAM and the CPE.

The table below describes the fields that the router uses for the various encapsulation types when calculating ATM overhead.

Table 1 Overhead Calculation

Encapsulation Type

Number of Bytes

Description

802.1Q

18

6-byte destination MAC address + 6-byte source MAC address + 2-byte protocol ID (0x8100) + 2-byte VID/CFI/PRIORITY + 2-byte length/type

802.3

14

6-byte destination MAC address + 6-byte source MAC address + 2-byte protocol ID (0x8000)

AAL5 MUX plus 1483

8

8-byte AAL5 trailer

AAL5 MUX plus PPPoA

10

8-byte AAL5 trailer + 2-byte protocol ID (0x002

AAL5 SNAP plus 1483

18

8-byte AAL5 trailer + 3-byte LLC header (0xAAAA03) + 3-byte OUI (0x0080c2) + 2-byte protocol ID (0x0007) + 2-byte PAD (0x0000)

AAL5 SNAP plus PPPoA

12

8-byte AAL5 trailer + 3-byte LLC header (0xFEFE03) + 1-byte protocol ID (0xCF)

PPPoE

6

1-byte version/type (0x11) + 1-byte code (0x00) + 2-byte session ID + 2-byte lengt

qinq

22

6-byte destination MAC address + 6-byte source MAC address + 2-byte protocol ID (0x8100) + 2-byte VID/CFI/PRIORITY + 2-byte protocol ID + 2-byte inner tag + 2-byte length or type

Overhead Accounting and Hierarchical Policies

In hierarchical policies, you can enable overhead accounting for shaping and bandwidth on top-level parent policies, middle-level child policies, and bottom-level child policies. If you enable overhead accounting on a:

  • Parent class-default class, then you are not required to enable accounting on a child traffic class that does not contain the bandwidth or shape command.
  • Child policy, then you must enable overhead accounting on the parent policy.

The parent and child classes must specify the same encapsulation type when enabling overhead accounting and configuring an offset using the user-defined offset [atm] command option.

The table below summarizes the configuration requirements for overhead accounting. For example, if overhead accounting is currently enabled for a parent policy, then accounting can be disabled or enabled on a child policy.

Table 2 Overhead Accounting Configuration Requirements

Policy Map or Class

Current Configuration

Configuration Requirement

Parent

Enabled

Enabled on child policy

Child

Enabled

Enabled on parent policy

Child class

Enabled

Enabled on all classes in the child policy map, except priority classes with policing

Child class (nonpriority without policing)

Disabled

Disabled on all classes in the child policy map

Child class (priority with policing)

Disabled

Disabled or enabled on all nonpriority classes in the child policy map

How to Configure Ethernet Overhead Accounting

Configuring Ethernet Overhead Accounting in a Hierarchical Policy

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 [atm]}]

6.    exit

7.    policy-map policy-map-name

8.    class class-default

9.    shape [average] rate[account {{qinq | dot1q} [aal5] {subscriber-encap}} | {user-defined offset [atm]}]

10.    service-policy policy-map-name

11.    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. Enters policy-map configuration mode.

  • The policy-map-name argument represents the name of the child policy map.
 
Step 4
class class-map-name


Example:

Router(config-pmap)# class video

 

Assigns the traffic class you specify to the policy map. Enters policy-map class configuration mode.

  • The class-map-name argument represents the name of a previously configured class map.
 
Step 5
bandwidth {bandwidth-kbps | percent percentage | remaining percent percentage} [account {{qinq | dot1q} {aal5} {subscriber-encapsulation}} | {user-defined offset [atm]}]


Example:

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

 

Enables class-based fair queueing and overhead accounting.

  • bandwidth-kbps--Specifies or modifies the minimum bandwidth allocated for a class belonging to a policy map. Valid values are from 8 to 2,488,320, which represents from 1 to 99 percent of the link bandwidth.
  • percentage--Specifies or modifies the maximum percentage of the link bandwidth allocated for a class belonging to a policy map. Valid values are from 1 to 99.
  • remaining percentage--Specifies or modifies the minimum percentage of unused link bandwidth allocated for a class belonging 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 Configuring Ethernet Overhead Accounting in a Hierarchical Policy.
  • user-defined--Indicates that the router is to use the offset value that you specify when calculating ATM overhead.
  • offset--Specifies the number of bytes that the router is to use when calculating overhead. Valid values are from -63 to 63 bytes.
  • atm--(Optional) Applies the ATM cell tax in the ATM overhead calculation.
 
Step 6
exit

Example:

router(config-pmap-c)# exit

 

Exits policy-map class configuration mode.

 
Step 7
policy-map policy-map-name


Example:

Router(config-pmap)# policy-map Test

 

Creates or modifies the top-level parent policy.

  • policy-map-name--Specifies the name of the parent policy map.
 
Step 8
class class-default


Example:

Router(config-pmap)# class class-default

 

Configures or modifies the parent class-default class.

 
Step 9
shape [average] rate[account {{qinq | dot1q} [aal5] {subscriber-encap}} | {user-defined offset [atm]}]


Example:

Router(config-pmap-c)# shape 8000 account qinq aal5 snap-dot1-rbe

 

Shapes traffic to the indicated bit rate and enables overhead accounting.

  • average (Optional)--Is the committed burst (Bc) that specifies the maximum number of bits sent out in each interval. This option is only supported on the PRE3.
  • rate--Indicates the bit rate used to shape the traffic, in bits per second. When this command is used with backward explicit congestion notification (BECN) approximation, the bit rate is the upper bound of the range of bit rates that are permitted.
  • 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-encap--Specifies the encapsulation type at the subscriber line. For more information, see the Configuring Ethernet Overhead Accounting in a Hierarchical Policy.
  • user-defined--Indicates that the router is to use the offset value you specify when calculating ATM overhead.
  • offset--Specifies the number of bytes the router is to use when calculating overhead. Valid values are from -63 to +63 bytes. The router configures the offset size if you do not specify the offset option.
  • atm--Applies the ATM cell tax in the ATM overhead calculation.
Configuring both the offset and atm options adjusts the packet size to the offset size and then adds the ATM cell tax.  
Step 10
service-policy policy-map-name


Example:

Router(config-pmap-c)# service-policy policy-map-name

 

Applies a child policy to the parent class-default class.

policy-map-name--Specifies the name of a previously configured child policy map.

Note    Do not specify the input or output keywords when applying a child policy to a parent class-default class.
 
Step 11
end


Example:

Router(config-pmap-c)# end

 

 

Verifying Overhead Accounting

Configuration Examples for Ethernet Overhead Accounting

Example Enabling Ethernet Overhead Accounting

The following configuration example shows how to enable Ethernet overhead accounting. In the example, the configuration of the policy map named ethernet_ovrh shapes class-default traffic at a rate of 200,000 kbps and enables overhead accounting with a user-defined value of 18. The ethernet_ovrh policy is attached to Gigabit Ethernet subinterface 1/0/0.100, thereby enabling overhead accounting on the subinterface.

Router# configure-terminal
Enter configuration commands, one per line. End with CNTL/Z.
Router(config)# policy-map ethernet_ovrh
Router(config-pmap)# class class-default
Router(config-pmap-c)# shape average 200000 account user-defined 18
!
Router(config)# interface GigabitEthernet1/0/0.100
Router(config-subif)# service-policy output ethernet_ovrh
!
Router# show running-config | begin 1/0/0.100
interface GigabitEthernet1/0/0.100
encapsulation dot1Q 101
pppoe enable group group_pta
service-policy output ethernet_ovrh

Example Verifying Ethernet Overhead Accounting

The following partial sample 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 10 account dot1q aal5 snap-rbe

Example Verifying Ethernet Overhead Accounting with User-Defined Option

The following sample output for the policy map named ethernet_ovrh indicates that Ethernet overhead accounting is enabled for shaping and that the user-defined offset is 18 bytes. The sample output from the showpolicy-mapinterface command indicates that the ethernet_ovrh policy map is attached to the Gigabit Ethernet subinterface 1/0/0.100, enabling overhead accounting on the subinterface.

Router# show policy-map ethernet_ovrh
Policy Map ethernet_ovrh
Class class-default
Average Rate Traffic Shaping
cir 200000 (bps) account user-defined 18
Router# show policy-map interface GigabitEthernet1/0/0.100
GigabitEthernet1/0/0.100
Service-policy output: ethernet_ovrh
Class-map: class-default (match-any)
0 packets, 0 bytes
30 second offered rate 0 bps, drop rate 0 bps
Match: any
0 packets, 0 bytes
30 second rate 0 bps
Queueing
queue limit 8 packets
(queue depth/total drops/no-buffer drops) 0/0/0
(pkts output/bytes output) 0/0
shape (average) cir 200000, bc 800, be 800
target shape rate 200000
Overhead Accounting Enabled

Additional References

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

Policing and Shaping.

"Policing and Shaping Overview" module

Class maps

"Applying QoS Features Using the MQC" module

Policy maps

"Applying QoS Features Using the MQC" module

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

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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:

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

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

Feature Information for Ethernet Overhead Accounting

The following table provides release information about the feature or features described in this module. This table lists only the software release that introduced support for a given feature in a given software release train. Unless noted otherwise, subsequent releases of that software release train also support that feature.

Use Cisco Feature Navigator to find information about platform support and Cisco software image support. To access Cisco Feature Navigator, go to www.cisco.com/go/cfn. An account on Cisco.com is not required.

Table 3 Feature Information for Ethernet Overhead Accounting

Feature Name

Releases

Feature Information

Ethernet Overhead Accounting

Cisco IOS XE Release 2.4

The Ethernet Overhead Accounting feature was introduced on the Cisco ASR 1000 series routers.

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