Per-VC Queueing on the PA-A3-8T1/E1 IMA ATM Port Adapter

Document ID: 10484

Contents

Introduction
Prerequisites
Supported Versions
Sample Configuration
show and debug Commands
Related Information

Introduction

Prerequisites

This document requires an understanding of IP to ATM CoS. The following resources provide information on these features.

• IP to ATM Class of Service Overview
• IP to ATM Class of Service Technology Support Page

Supported Versions

As of Cisco IOS Software Releases 12.1(5)T, 12.1(5)E, and 12.0(14)S, only VIP-based or distributed per-VC queueing is supported on the 7500 Series. RSP-based QoS is no longer supported. Configuring a service policy with distributed LLQ (dLLQ) in a Cisco IOS image without dLLQ support leads to VIP crashes (CSCdt78618).

Sample Configuration

Our traffic generator sent two types of traffic, one emulating voice and the other "disturbing" data to congest the VC's hardware or driver-level first-in-first-out (FIFO) queue. By virtue of LLQ, the voice traffic received the expected priority.

When configuring LLQ for voice traffic, ensure that you decrease the size of the hardware FIFO queue, known as the transmit ring, with the tx-ring-limit command. Otherwise, the voice traffic may experience excessive latency.  Normally, a TX-ring-limit value of three or four is fine. Using this sample configuration, voice latency averaged 7ms, compared to 3ms in the absence of competing traffic.
 

LLQ on a PA-A3-8T1IMA

class-map match-all rtp    match ip rtp 5000 10000 ! policy-map llq    class rtp      priority 1000 ! interface ATM3/0   no ip address   ima-group 0 ! interface ATM3/ima0 no ip address no atm ilmi-keepalive ! interface ATM3/ima0.34 point-to-point   ip address 5.0.34.3 255.255.255.0   pvc 0/34   tx-ring-limit 3    vbr-nrt 4000 4000    service-policy output llq ! interface ATM3/1   no ip address   ima-group 0 ! interface ATM3/2   no ip address   ima-group 0

Note: All commands entered in VC configuration mode take effect only when you exit for this mode. Exiting causes the VC to be removed and recreated with the new configuration parameters, which may include QoS policies, shaping rates, etc. If you wish to remove a QoS service policy, you must type exit or CTRL-Z to ensure that the no form of the command actually takes effect.

show and debug Commands

• show queueing int atm and show queue int atm - Display the flows inside the queue. These commands only apply to the 7200 Series. To display flow information on the 7500 Series, enable NetFlow.
• show policy-map - Displays any configured queueing strategies.

• show policy-map interface - Displays the queueing statistics of an interface or a VC. For guidance on how to interpret the output
  from this command, see Understanding Packet Counters in show policy-map interface Output.

     r3#show policy-map interface atm3/ima0.34 
       ATM3/ima0.34: VC 0/34 -    

   Service-policy output: llq (1089)    

     Class-map: rtp (match-all) (1091/2) 
            2242843 packets, 435111542 bytes      
            ! -- Number of packets matching the criteria of the class. 
            30 second offered rate 775000 bps, drop rate 0 bps 
            Match: ip rtp 5000 10000 (1095)      
            Weighted Fair Queueing 
              Strict Priority      
              Output Queue: Conversation 136 
              Bandwidth 1000 (kbps) Burst 25000 (Bytes) 
              (pkts matched/bytes matched) 2016845/391267930 
              ! -- Number of packets matching the class during periods 
              ! -- of congestion, when the interface's transmit ring was full.      
              (total drops/bytes drops) 0/0    

     Class-map: class-default (match-any) (1099/0)      
            3987455 packets, 6028018316 bytes      
            30 second offered rate 8051000 bps, drop rate 0 bps 
            Match: any  (1103) 

Related Information

• ATM Technology Support Pages

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