Applying QoS Features to Ethernet Subinterfaces

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Updated:August 10, 2005
Document ID:10104

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Introduction

This document describes how to apply class-based weighted fair queueing (CBWFQ) and other Cisco IOS® Software-based Quality of Service (QoS) features on an Ethernet subinterface. An Ethernet subinterface is a logical interface in Cisco IOS. You can use the modular QoS command-line interface (CLI) (MQC) to create and apply a service policy to an Ethernet subinterface.

Prerequisites

Requirements

There are no specific requirements for this document.

Components Used

The information in this document is based on these software and hardware versions:

  • Cisco IOS Software 12.2(2)T

  • Cisco 2620 router with a Fast Ethernet network module

The information presented in this document was created from devices in a specific lab environment. All of the devices used in this document started with a cleared (default) configuration. If you are working in a live network, ensure that you understand the potential impact of any command before using it.

Conventions

For more information on document conventions, refer to Cisco Technical Tips Conventions.

Applying a Service Policy

In general, choosing where to apply a policy depends on the QoS features that your policy is activating. An Ethernet subinterface supports the following:

  • Class-based policing - If you apply a policy with the police command to both the interface and the subinterface, only the subinterface policer is active for the traffic that matches the class. Refer to Traffic Policing for more information.

  • Class-based marking—refer to Classification Overview for more information.

  • Class-based shaping—refer to Configuring Class-Based Shaping for more information.

  • Class-based queueing—Queueing is a special case for Ethernet subinterfaces. See the remainder of this section for more information.

A router begins to queue packets when the number of packets that needs to be transmitted out an interface exceeds the output rate of that interface. The excess packets are then queued. A queueing method can be applied to packets waiting to be transmitted.

Cisco IOS logical interfaces do not inherently support a state of congestion and do not support the direct application of a service policy that applies a queueing method. Instead, you first need to apply shaping to the subinterface using either generic traffic shaping (GTS) or class-based shaping. Refer to Policing and Shaping for more information.

The router prints this log message when an Ethernet subinterface is configured with a service policy that applies queueing without shaping: 

router(config)# interface ethernet0/0.1
router(config-subif)# service-policy output test
 CBWFQ : Not supported on subinterfaces

Note that the same rule applies to a Gigabit Ethernet subinterface. 

c7400(config)# interface gig0/0.1
c7400(config-subif)# service-policy ou
c7400(config-subif)# service-policy output outFE
  CBWFQ : Not supported on subinterfaces

In other words, you must configure a hierarchical policy with the shape command at the parent level. Use the bandwidth command for CBWFQ, or the priority command for Low Latency Queueing (LLQ) at lower levels. Class-based shaping limits the output rate and (we can assume) leads to a congested state on the logical subinterface. The subinterface than applies "backpressure," and Cisco IOS begins queueing the excess packets that are held by the shaper.

Applying a Hierarchical Policy

Follow these steps to apply a hierarchical policy:

  1. Create a child or lower-level policy that configures a queueing mechanism. In the example below, we configure LLQ using the priority command and CBWFQ using the bandwidth command. Refer to Congestion Management Overview for more information. 

    policy-map child 
 class voice 
  priority 512

  2. Create a parent or top-level policy that applies class-based shaping. Apply the child policy as a command under the parent policy since the admission control for the child class is done based on the shaping rate for the parent class. 

    policy-map parent
 class class-default 
  shape average 2000000 
  service-policy child

  3. Apply the parent policy to the subinterface. 

    interface ethernet0/0.1
 service-policy parent

Configuring Class-Based Shaping

In this section, you are presented with the information to configure the features described in this document.

Note: To find additional information on the commands used in this document, use the Command Lookup Tool (registered customers only) .

Configuration

Router 2620A 
hostname 2620A 
! 
ip cef 
! 
class-map match-any dscp46 
 match ip dscp 46 
class-map match-all telnet_ping_snmp 
 match access-group 150 
class-map match-all http 
 match access-group 154 
class-map match-all pop3_smtp 
 match access-group 153 
! 
! 
policy-map voice_traffic 
 class dscp46 
  shape average 30000 10000 
 class telnet_ping_snmp 
  shape average 20000 15440 
 class pop3_smtp 
  shape average 20000 15440 
 class http 
  shape average 20000 15440 
! 
interface FastEthernet0/0 
 ip address 10.10.247.2 255.255.255.0 
 duplex auto 
 speed auto 
! 
interface FastEthernet0/0.1 
 encapsulation dot1Q 1 native 
 ip address 10.1.1.1 255.255.255.0 
 service-policy output voice_traffic

Verify

This section provides information you can use to confirm your configuration is working properly.

Certain show commands are supported by the Output Interpreter tool, which allows you to view an analysis of show command output.

  • show policy-map {policy name} - Displays the configuration of all classes for a specified service policy map. 

    2620A# show policy-map voice_traffic 
Policy Map voice_traffic 
Class dscp46 
Traffic Shaping 
Average Rate Traffic Shaping 
CIR 30000 (bps) Max. Buffers Limit 1000 (Packets) 
Bc 10000 
Class telnet_ping_snmp 
Traffic Shaping 
Average Rate Traffic Shaping 
CIR 20000 (bps) Max. Buffers Limit 1000 (Packets) 
Bc 15440 
Class pop3_smtp 
Traffic Shaping 
Average Rate Traffic Shaping 
CIR 20000 (bps) Max. Buffers Limit 1000 (Packets) 
Bc 15440 
Class http 
Traffic Shaping 
Average Rate Traffic Shaping 
CIR 20000 (bps) Max. Buffers Limit 1000 (Packets) 
Bc 15440 

2620A# show policy-map voice_traffic class dscp46 
Class dscp46 
Traffic Shaping 
Average Rate Traffic Shaping 
CIR 30000 (bps) Max. Buffers Limit 1000 (Packets) 
Bc 10000

  • show policy-map interface fast - Displays match counters for all classes of a specified service policy map. 

    2620A# show policy-map interface fa0/0.1 
FastEthernet0/0.1 
Service-policy output: voice_traffic 
Class-map: dscp46 (match-any) 
0 packets, 0 bytes 
5 minute offered rate 0 bps, drop rate 0 bps 
Match: ip dscp 46 
0 packets, 0 bytes 
5 minute rate 0 bps 
Traffic Shaping 
Target    Byte   Sustain   Excess    Interval  Increment Adapt 
Rate      Limit  bits/int  bits/int  (ms)      (bytes)   Active 
30000     2500   10000     10000     333       1250      - 
Queue     Packets   Bytes     Packets   Bytes     Shaping 
Depth                         Delayed   Delayed   Active 
0         0         0         0         0         no 
Class-map: telnet_ping_snmp (match-all) 
0 packets, 0 bytes 
5 minute offered rate 0 bps, drop rate 0 bps 
Match: access-group 150 
Traffic Shaping 
Target    Byte   Sustain   Excess    Interval  Increment Adapt 
Rate      Limit  bits/int  bits/int  (ms)      (bytes)   Active 
20000     3860   15440     15440     772       1930      - 
Queue     Packets   Bytes     Packets   Bytes     Shaping 
Depth                         Delayed   Delayed   Active 
0         0         0         0         0         no 
Class-map: pop3_smtp (match-all) 
0 packets, 0 bytes 
5 minute offered rate 0 bps, drop rate 0 bps 
Match: access-group 153 
Traffic Shaping 
Target    Byte   Sustain   Excess    Interval  Increment Adapt 
Rate      Limit  bits/int  bits/int  (ms)      (bytes)   Active 
20000     3860   15440     15440     772       1930      - 
Queue     Packets   Bytes     Packets   Bytes     Shaping 
Depth                         Delayed   Delayed   Active 
0         0         0         0         0         no 
Class-map: http (match-all) 
0 packets, 0 bytes 
5 minute offered rate 0 bps, drop rate 0 bps 
Match: access-group 154 
Traffic Shaping 
Target    Byte   Sustain   Excess    Interval  Increment Adapt 
Rate      Limit  bits/int  bits/int  (ms)      (bytes)   Active 
20000     3860   15440     15440     772       1930      - 
Queue     Packets   Bytes     Packets   Bytes     Shaping 
Depth                         Delayed   Delayed   Active 
0         0         0         0         0         no 
Class-map: class-default (match-any) 
926 packets, 88695 bytes 
5 minute offered rate 0 bps, drop rate 0 bps 
Match: any

Note: Class-based shaping works at the interface and subinterface level. Cisco IOS 12.2(2.5) introduces the ability to configure shaping on the main interface and IP addresses on the subinterfaces.

Related Information

Revision History

Revision Publish Date Comments
1.0
10-Aug-2005
Initial Release

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