ATM Traffic Management

Document ID: 15992

Contents

Introduction
Prerequisites
      Requirements
      Components Used
      Conventions
Shaped and Non-Shaped PVCs
Configure WS-X515x and WS-5166 Modules
      Configure a CBR VC
      Configure a UBR VC
      Understand Rate Queues and Scheduling
Configure Shaping and Queueing on WS-X516x Modules
Configure Shaping and Queueing on the WS-X6101
Throttle an Interface with the atm traffic-shape rate Command
Configure the traffic-shape rate Command
Related Information

Introduction

This document summarizes support for ATM-layer quality of service (QoS). This includes traffic shaping and rate queues on the Layer 2 ATM modules for the Catalyst 5000 and Catalyst 6000 Series. These modules support both permanent virtual circuits (PVCs) and LAN emulation (LANE). This document discusses QoS on PVCs only. For more information on LANE QoS, which includes how to configure UBR+ virtual circuit connections (VCCs), refer to Configuring LANE QoS.

Prerequisites

Requirements

There are no specific requirements for this document.

Components Used

This document is not restricted to specific software and hardware versions.

Conventions

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

Shaped and Non-Shaped PVCs

The ATM Forum publishes standards for ATM technology. One such standard, the Traffic Management Specification Version 4.0, defines five service categories that describe transmitted user traffic and the QoS that a network needs in order to provide for that traffic. These are the five service categories:

• CBR

• VBR-nrt

• VBR-rt

• ABR

• UBR and UBR+

The ATM modules for the Catalyst 5000 and 6000 support a subset of these categories. This support depends on the particular line card. This table describes such support.

Configure WS-X515x and WS-5166 Modules

The WS-X515x Series and the WS-X5166 support two images. The first image supports LANE and non-shaped PVCs and the second image supports shaped PVCs. This table lists the images that support shaped PVCs.

Note: The first traffic-shaping image of the Cisco IOS® Software Release train 12.0W5 is 12.0(10)W5(18a). With this release, the name of the traffic shaping image changes to c5atm-wt-mz.

Configure a CBR VC

The WS-X515x Series and the WS-X5166 support the CBR service category. In order to create a CBR VC, specify a value for the PCR. The initial traffic-shaping image (Software Release 51.1) allows you to configure SCR and MBS values even though the image supports only PCR-based shaping. Current images in the Cisco IOS Software Release 11.3 and 12.0 trains support the configuration of a PCR value only, as shown in this output, for an ATM module that runs Cisco IOS Software Release 12.0(10)W5(18a).

atm(config-subif)#atm pvc 1000 1 100 aal5snap ? 
    <1-155000> Peak rate(Kbps)    
    oam OAM loopback    enable 
    <cr>
   atm#show atm vc 1000 
   atm0.100: VCD: 1000, VPI: 1, VCI: 100 
   PeakRate: 100000, Average Rate: 0, Burst Cells:    0 
   AAL5-LLC/SNAP, etype:0x0, Flags: 0x820, VCmode: 0x1 
   OAM frequency: 0 second(s) 
   InARP DISABLED 
   InPkts: 0, OutPkts: 0 
   InPRoc: 0, OutPRoc: 0 
   InFast: 0, OutFast: 0, InAS: 0, OutAS: 0 
   OAM cells received: 0 
   OAM cells sent: 0 
   Status: UP

Configure a UBR VC

On any ATM interface, from Catalyst ATM modules to router port adapters, if you create a PVC without any specific shaping parameters, it creates a non-shaped PVC at the line rate. The creation of a shaped PVC means that you specify a non-zero value for the PCR or the SCR. This depends on which module you use.

In this example, two PVCs are created. PVC 100 is a shaped PVC with a peak rate of 100 Mbps. PVC 101 is a non-shaped PVC with a peak rate of the physical line rate. Use the show atm vc vcd# command in order to view the traffic parameters of each PVC.

interface atm0 
   logging event subif-link-status
   atm vc-per-vp 128 
   atm preferred phy A 
   atm pvc 1 0 5 qsaal 
   atm pvc 2 0 16 ilmi 
   no atm ilmi-keepalive 
   ! 
   interface atm0.5 point-to-point
   atm pvc 100 5 100 aal5snap 100000 
   ! 
   interface atm0.6 point-to-point 
   atm pvc 101 5 101 aal5snap

   atm#show atm vc 100 
   atm0.5: VCD: 100, VPI: 5, VCI: 100 
   PeakRate: 100000, Average Rate: 0, Burst Cells: 0 
   AAL5-LLC/SNAP, etype:0x0, Flags: 0x820, VCmode: 0x1 
   OAM frequency: 0 second(s) 
   InARP DISABLED 
   InPkts: 0, OutPkts: 0 
   InPRoc: 0, OutPRoc: 0 
   InFast: 0, OutFast: 0, InAS: 0, OutAS: 0 
   OAM cells received: 0 
   OAM cells sent: 0 
   Status: UP

   atm#show atm vc 101 
   atm0.6: VCD: 101, VPI: 5, VCI: 101 
   PeakRate: 155000, Average Rate: 0, Burst Cells: 0 
   AAL5-LLC/SNAP, etype:0x0, Flags: 0xC20, VCmode: 0x201 
   OAM frequency: 0 second(s) 
   InARP DISABLED 
   InPkts: 0, OutPkts: 0 
   InPRoc: 0, OutPRoc: 0 
   InFast: 0, OutFast: 0, InAS: 0, OutAS: 0 
   OAM cells received: 0 
   OAM cells sent: 0 
   Status: UP


atm#show atm vc 
              VCD /                            Peak Avg/Min Burst 
   Interface  Name   VPI  VCI   Type  Encaps   Kbps  Kbps Cells Sts 
   0          1        0    5   pvc   SAAL        0     0      INAC 
   0          2        0   16   pvc   ILMI        0     0      INAC 
   0.5        
100
      5  100   pvc   SNAP   100000     0   0    UP 
   0.6        101      5  101   pvc   SNAP   155000     0   0    UP 
 

Understand Rate Queues and Scheduling

The WS-X515x Series and the WS-X5166 use rate queues, which are created dynamically as you configure PVCs with shaping values. In other words, each rate queue supports VCs of a particular shaping value. Up to ten such queues are available.

The number of queues per range of supported PCR values varies between the Digital Signal Level 3 (DS-3) and the Optical Carrier Level 3 (OC-3) ATM modules. This table describes these values:

Note: The configuration of more than two rate queues with PCR values less than 2080 Kbps for OC-3 is not possible. An example of that uses the Catalyst switch 5500 and WS-X5157 card is shown in this output:

			atm(config)#interface atm 0 
   atm(config-if)#atm pvc 3 2 34 aal5snap 2000 oam 3
   atm(config-if)#atm pvc 4 2 35 aal5snap 128 oam 3
   atm(config-if)#atm pvc 5 2 36 aal5snap 1000 oam 3
   
			Not creating vc:5 interface: ATM0 is out of rate queues.

This example attempts to configure three rate queues (2000 Kbps,128 Kbps and 1000 Kbps). The workaround for this limitation of the ability to only configure two rate queues is to configure the first rate queue at 2081 kbps or higher (instead of 2000 Kbps) so that it falls in the PCR range of 2081 to 155,000 Kbps. In this way you have two other rate queues available for 128 and 1000.

In Cisco IOS Software Releases prior to 12.0(16)W5(21), the show atm interface atm0 command did not display the rate queues with PCR values less than 2081 kbps for OC-3 interfaces and less than 2354 kbps for DS3 interfaces. This issue is resolved in current Cisco IOS Software Releases. If you use an earlier Cisco IOS Software Release, use the write terminal command in order to display the VCs configured for these special rate queues.

The WS-X515x Series and the WS-X5166 use Weighted Round Robin (WRR) scheduling which depends on the PCR. WRR is similar to round robin with the difference that each rate-queue is assigned a unique weight which depends on the PCR. The weight then increases or decreases the frequency with which the rate-queue is scheduled.

They do not consider the number of PVCs with the same rate for weight age. As a result, if you configure many PVCs with a high rate and only a few PVCs with a low rate, the low-rate VCs can get a greater throughput rate. The reason is that the scheduler does not assign a weight age to the number of PVCs with the same rate. The WRR scheduling depends purely on the configured PCRs. This table shows an example to help explain this concept:

75 / 10 Mbps = 7.5

25 / 2 Mbps = 12.5 Mbps 

This example shows that the configuration of many VCs with the same PCR value affects the output rate of each VC. In other words, each of the 25 Mbps PVCs in this example receive higher throughput than each of the 75 Mbps PVCs. Cisco recommends that you distribute your PVCs equally across all ten rate queues in order to ensure maximum throughput.

Use the show atm interface atm0 command in order to display the currently active rate queues and the number of PVCs in each queue. In this sample output, Rate-Queue 0 has one shaped VCC with a PCR of 100 Mbps, and Rate-Queue 1 has two non-shaped VCCs with PCR set to the physical line rate.

You can monitor the rate queues with the show atm interface atm0 command (the output does not correspond to the example which precedes):

atm#show atm interface atm0 
   atm interface atm0: 
   AAL enabled: AAL5 , Maximum VCs: 4096, Current VCCs: 3 
   Maximum Transmit Channels: 0 
   Max. Datagram Size: 1580 
   PLIM Type:SONET - 155Mbps, TX clocking: LINE 
   0 input, 12 output, 0 IN fast, 0 OUT fast 
   Rate-Queue 0 set to 100000Kbps, reg=0x0 DYNAMIC, 1 VCC 
   Rate-Queue 1 set to 155000Kbps, reg=0x0 DYNAMIC, 2 VCCs 
   Config. is ACTIVE

Configure Shaping and Queueing on WS-X516x Modules

The WS-X516x Series (all modules other than the WS-X5166) supports the CBR, VBR-nrt, and UBR ATM service categories. In order to create a CBR PVC, configure PCR=SCR. (You cannot enter a value of 0 or leave SCR unconfigured.) In order to create a UBR VC, enter the virtual path identifier (VPI) and virtual channel identifier (VCI) values and do not specify any shaping parameters.

In order to configure a VBR-nrt VC on these modules, use this command:

atm pvc vcd vpi vci [aal5snap | ilmi | qsaal] [peak_rate average_rate] [burst_size] [oam seconds]

These ATM modules support a particular set of PCR and SCR values. When you configure any other values, the Catalyst ATM software rounds down and chooses the next lowest supported value. Thus, the values you specify at the command-line can differ from the actual values that the module applies to the PVC.

The official set of values is derived from this formula:

Output Rate = Line Rate / N

In this equation, N is an integer or whole number without fractions or decimals. Line Rate is a full 155 Mbps for the OC-3 and a full 45 Mbps for the DS-3. Use the show atm interface atm0 command in order to view the line rate.

console> show atm interface atm0 
    atm interface atm0: 
    AAL enabled: AAL5 , Maximum VCS: 4096, Current VCCs:    12 
    Tx buffers 32, Rx buffers 32, Exception Queue: 32, Raw    Queue: 32 
    VP Filter: 0x0, VCIs per VPI: 1024, Max. Datagram Size:1580    
    PLIM Type:SONET - 155Mbps,    TX clocking: LINE 
    1323970 input, 1135820 output, 0 IN fast, 0 OUT fast    
    Config. Is ACTIVE 
    console>

In this example, you need to configure six queues at the rate of 100M, 50M, 30M, 10M and 1M on an OC-3 module. With the Output Rate = Line Rate / N formula, you can derive the actual values:

100M: 155/2 = 77.5 
   50M: 155/3=51.67 or 38.75 
   30M: 155/5 = 31 or 155/6 25.8 
   10M: 155/15 = 10.33 or 9.69 
   5M: 155/31 = 5 
   1M: 155/155 = 1

The WS-X516x Series (other than the WS-X5166) also uses WRR in order to schedule frames from the VCCs. It assigns a default (non-configurable) transmit priority which depends on the ATM service category, such as CBR, VBR, and UBR. When it calculates the weight, the scheduler considers the SCR of an individual PVC. It then provides a higher transmission rate for PVCs with higher SCR values.

If you choose to configure the SCR values of your VBR-nrt VCs such that you oversubscribe the line rate, the Catalyst ATM module accepts this configuration and does not print an error or warning indication. In the case of oversubscription, packets are dropped in proportion to the rate, such as PCR for a CBR VC and SCR for a VBR VC, across all the PVCs.

On these modules, there is no rate queue concept and no limit on the number of rates you can configure. Queues of the same shaping parameters are not placed in the same queue. Instead, each PVC maintains a packet queue. Queues are created as VCs are created. Thus, the output of the show atm interface atm0 command does not display any rate queues.

Configure Shaping and Queueing on the WS-X6101

The WS-X6101 supports the VBR-nrt and UBR ATM service categories. The SAR assigns a lower internal priority value to UBR VCs than VBR-nrt VCs. The SAR first schedules a cell from a VBR-nrt VC in an available cell timeslot. If no VBR VCs have data for transmission or if the interface supports a single VC, then the entire link bandwidth is available for the UBR VC. If the network leg of your PVC is provisioned as CBR, you can configure CBR-like PVCs on the WS-X6101 by the configuration of a VBR-nrt VC with PCR = SCR. With this configuration, the CBR-like VC is given a higher SAR scheduling priority than UBR VCs.

If a particular PVC is provisioned as CBR or VBR-rt in the ATM network in order to support real-time traffic like VoIP and you need to provide equivalent support on the WS-X6101, Cisco recommends one of these approaches:

• The SAR on the WS-X6101 schedules cells at a constant inter-cell gap. Thus, it maintains very low jitter and consistent scheduling which the VoIP requires. In order to configure a CBR-like PVC, configure PCR=SCR, as shown here:

  atm(config-if)#atm pvc 35 0 35 aal5 10000 10000 
  

  Although such CBR-like PVCs appear as UBR in the output of the show atm pvc vpi/vci command, they receive a higher priority than UBR PVCs. However, they share the same priority level as VBR-nrt VCs. (In other words, shaped PVCs (CBR and VBR) are given higher priority over unshaped PVCs (UBR). The WS-X6101 modules are Layer 2 only and do not support IP to ATM Class of Service (CoS) features, such as class-based weighted fair queueing (CBWFQ) and low latency queueing (LLQ).

• Use a FlexWAN module with a PA-A3. The PA-A3 supports four priority levels, which you can tune for each VC with the transmit-priority command. In addition, the PA-A3 supports IP to ATM CoS. Refer to Configuring Per-VC Queueing and IP to ATM CoS on the PA-A3 and FlexWAN.

For further information on how the maintenance of a constant inter-cell gap in SAR scheduling provides equivalent CBR-like performance, refer to Understanding Router Support for ATM Real-Time Service Categories.

Throttle an Interface with the atm traffic-shape rate Command

In addition to shaping at the PVC level, you also can shape at the interface level with the atm traffic-shape rate command. Specify any whole number from 1 to 155.

This example shows how to throttle or limit the output of an ATM interface to 50 Mbps:

atm(config-if)#atm traffic-shape rate 50

This command can be useful if you must use the LANE and non-shaping Cisco IOS image, such as c5atm-wl-mc.120-12.W5.19.bin, on the WS-X515x Series and the WS-X5166. Otherwise, Cisco recommends that you specify a PCR value with the atm pvc command in order to use native ATM shaping.

Configure the traffic-shape rate Command

In Cisco IOS Software Releases prior to 12.0(16)W5(21), the command-line interface (CLI) on the Catalyst ATM modules allows you to configure the traffic-shape rate command. This command implements generic traffic shaping (GTS), which is a Cisco IOS feature available for many interface types. Although you can see matches in the packets and bytes columns of the show traffic-shape statistics command if you configure GTS, the ATM modules never supported GTS. The traffic-shape rate command no longer appears at the CLI in current releases of Cisco IOS Software for the Catalyst ATM modules.

Cisco recommends that you configure native ATM shaping in order to avoid unexpected cell drops in an ATM network cloud, particularly if the provider implements policing. For more information on the differences between GTS and native ATM shaping, refer to the discussion of this issue in Does the PA-A1 ATM Port Adapter Support Traffic Shaping?.

Related Information

• Traffic Shaping on ATM Line Cards for the Cisco 12000
• Traffic-Shaping and Soft-VCs on the ATM Route Module (ARM)
• Configuring the ATM Traffic-Shaping Carrier Module
• ATM Technology Support
• Technical Support - Cisco Systems