Do Both Router Ends of an ATM PVC Need to Use the Same Traffic Shaping Values?

Introduction

In typical hub and spoke wide-area network topologies, the traffic flow volume is asymmetrical, in which more traffic flows down to the remote site than comes from the remote site. Such configurations may benefit from provisioning an asymmetrical permanent virtual circuit (PVC), which uses different peak cell rate (PCR) and sustained cell rate (SCR) traffic shaping values at the two router ends of a nonreal-time variable bit rate (nrt-VBR) PVC.

The purpose of this document is to provide a sample configuration for an asymmetrical PVC. Note that, if not properly implemented in the ATM switch network, the slower end of the PVC may experience cyclic redundancy check (CRC) errors, which may affect performance.

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.

Are PVCs Unidirectional?

An ATM virtual connection consists of two unidirectional sublinks. While the connection allocates the same virtual path identifier (VPI)/virtual channel identifier (VCI) in both directions, it can allocate different bandwidth amounts in each direction. Such a connection is referred to as asymmetrical.

Configuration

This sample configuration on the LS1010 ATM switch uses two routers with unique PCR=SCR values:

• Central site—7 MB PCR=SCR, connected to LS1010 port 0/0/0

• Remote site—3.5 MB PCR=SCR, connected to LS1010 port 1/0/0

atm connection-traffic-table-row index 63999 vbr-nrt pcr 3500 scr0 3500  mbs 200 
atm connection-traffic-table-row index 64000 vbr-nrt pcr 7000 scr0 7000  mbs 200 
! 
interface ATM1/0/0 
 no atm auto-configuration 
 no atm ilmi-keepalive 
 atm pvc 5 100 rx-cttr 63999 tx-cttr 64000  interface  ATM0/0/0 5 100

show Commands and Configuration Notes

Certain show commands are supported by the Output Interpreter Tool (registered customers only) , which allows you to view an analysis of show command output.

• show atm connection-traffic-table—Cisco campus ATM switches store traffic parameters for each connection in a separate table of connection traffic table row (CTTR) values. You first configure traffic parameter values and assign an index number to the row in the CTTR table. Issue the following command to view these CTTR rows:

  ls1010-2#show atm connection-traffic-table 
  Row      Service-category    pcr        scr/mcr       mbs         cdvt 
  1            ubr          7113539        none                     none 
  2            cbr              424                                 none 
  3            vbr-rt           424         424          50         none 
  4            vbr-nrt          424         424          50         none 
  5            abr              424           0                     none 
  6            ubr              424        none                     none 
  63997        cbr             1741                                 none 
  63998        vbr-rt          5000        2500-0       none        none 
  63999        vbr-nrt         3500        3500-0       200         none 
  64000        vbr-nrt         7000        7000-0       200         none 
  2147483645*  ubr                0        none                     none 
  2147483646*  ubr                1        none                     none 
  2147483647*  ubr          7113539        none                     none 

• The atm pvc command accepts tx-cttr and a rx-cttr parameters for each leg of the VC. So what direction is the transmit and what direction is the receive? The view is switch-fabric centric. In other words, rx is into the switch fabric from the interface, and tx is out of the switch fabric to the interface.

• There are multiple ways to achieve the same traffic-shaping result. In other words, these commands are equivalent:

  interface atm 1/0/0
  
  
    atm pvc 5 100 rx-cttr 63999 tx-cttr 64000 interface atm 0/0/0 5 100 
    atm pvc 5 100 interface atm 0/0/0 5 100 rx-cttr 64000 tx-cttr 63999 
    atm pvc 5 100 rx-cttr 63999 interface atm 0/0/0 5 100 rx-cttr 64000
  

• Issue the show atm vc interface x/y/z vpi vci command to confirm the asymmetrical parameters of the connection.

  0/0/0 - rx is 7MB 
  0/0/0 - tx is 3.5MB 1/0/0 - tx is 7MB 
  1/0/0 - rx is 3.5 MB 
  
  ls1010-2#show atm vc interface atm 0/0/0 5 100 
  
  Interface: ATM0/0/0, Type: oc3suni 
  VPI = 5  VCI = 100 
  Status: DOWN 
  Time-since-last-status-change: 1d14h 
  Connection-type: PVC 
  Cast-type: point-to-point 
  Packet-discard-option: disabled 
  Usage-Parameter-Control (UPC): pass 
  Wrr weight: 2 
  Number of OAM-configured connections: 0 
  OAM-configuration: disabled 
  OAM-states:  Not-applicable 
  Cross-connect-interface: ATM1/0/0, Type: ds3suni_Quad 
  Cross-connect-VPI = 5 
  Cross-connect-VCI = 100 
  Cross-connect-UPC: pass 
  Cross-connect OAM-configuration: disabled 
  Cross-connect OAM-state:  Not-applicable 
  Threshold Group: 3, Cells queued: 0 
  Rx cells: 0, Tx cells: 0 
  Tx Clp0:0,  Tx Clp1: 0 
  Rx Clp0:0,  Rx Clp1: 0 
  Rx Upc Violations:0, Rx cell drops:0 
  Rx Clp0 q full drops:0, Rx Clp1 qthresh drops:0 
  Rx connection-traffic-table-index: 64000 
  Rx service-category: VBR-NRT (Non-Realtime Variable Bit Rate) 
  Rx pcr-clp01: 7000 
  Rx scr-clp0 : 7000 
  Rx mcr-clp01: none 
  Rx      cdvt: 1024 (from default for interface) 
  Rx       mbs: 200 
  Tx connection-traffic-table-index: 63999 
  Tx service-category: VBR-NRT (Non-Realtime Variable Bit Rate) 
  Tx pcr-clp01: 3500 
  Tx scr-clp0 : 3500 
  Tx mcr-clp01: none 
  Tx      cdvt: none 
  Tx       mbs: 200 
  
  ls1010-2#show atm vc interface atm 1/0/0 5 100 
  
  Interface: ATM1/0/0, Type: ds3suni_Quad 
  VPI = 5  VCI = 100 
  Status: DOWN 
  Time-since-last-status-change: 1d14h 
  Connection-type: PVC 
  Cast-type: point-to-point 
  Packet-discard-option: disabled 
  Usage-Parameter-Control (UPC): pass 
  Wrr weight: 2 
  Number of OAM-configured connections: 0 
  OAM-configuration: disabled 
  OAM-states:  Not-applicable 
  Cross-connect-interface: ATM0/0/0, Type: oc3suni 
  Cross-connect-VPI = 5 
  Cross-connect-VCI = 100 
  Cross-connect-UPC: pass 
  Cross-connect OAM-configuration: disabled 
  Cross-connect OAM-state:  Not-applicable 
  Threshold Group: 3, Cells queued: 0 
  Rx cells: 0, Tx cells: 0 
  Tx Clp0:0,  Tx Clp1: 0 
  Rx Clp0:0,  Rx Clp1: 0 
  Rx Upc Violations:0, Rx cell drops:0 
  Rx Clp0 q full drops:0, Rx Clp1 qthresh drops:0 
  Rx connection-traffic-table-index: 63999 
  Rx service-category: VBR-NRT (Non-Realtime Variable Bit Rate) 
  Rx pcr-clp01: 3500 
  Rx scr-clp0 : 3500 
  Rx mcr-clp01: none 
  Rx      cdvt: 1024 (from default for interface) 
  Rx       mbs: 200 
  Tx connection-traffic-table-index: 64000 
  Tx service-category: VBR-NRT (Non-Realtime Variable Bit Rate) 
  Tx pcr-clp01: 7000 
  Tx scr-clp0 : 7000 
  Tx mcr-clp01: none 
  Tx      cdvt: none 
  Tx       mbs: 200 

• If not properly implemented, the slower receiving interface can experience input drops. Specifically, the PA-A3 allocates a subset of the total receive buffers to a VC based on its traffic shaping values. If a VC receives more packets than it expects due to an incorrectly implemented asymmetrical VC, these receive buffers fill. The router must drop subsequent packets and increments the InPktDrops counter in the show atm vc output. If your PVC logs InPktDrops, check the traffic-shaping configuration in the ATM switch network and at both router ends.

• The OC-3 and DS-3 network modules for the 2600 and 3600 series use a different architecture. These modules do not place a quota on the number of receive buffers allocated to a VC and thus should not report input drops if the VC is not configured properly in the ATM network.

Specifying Input and Output Values on a Router

An ATM router interface uses signaling protocols like Q.2931 to establish a switched virtual circuit (SVC). The signaling protocols start the SVC process by sending a SETUP message that includes multiple information elements (IEs). The purpose of IEs is to communicate the characteristics of the connection.

The traffic descriptor IE negotiates the traffic parameters of a connection, including forward and backward PCR, forward and backward SCR, and forward and backward maximum burst size (MBS). When configuring SVCs on an ATM router interface, the vbr-nrt command accepts input-pcr, input-scr, and input-mbs parameters. The full syntax of the command is:

vbr-nrt output-pcr output-scr output-mbs [input-pcr] [input-scr] [input-mbs]

In the following example, an output PCR and SCR of 5 MB is specified, as well as an input PCR and SCR of 2.5 MB.

7200-1.3(config-subif)#svc LESLIE nsap 
47.00918100000000E04FACB401.00E04FACB401.00
7200-1.3(config-if-atm-vc)#vbr-nrt 5000 5000 94 ? 
  <1-45000>  Input Peak Cell Rate(PCR) in Kbps 
  <cr> 

7200-1.3(config-if-atm-vc)#vbr-nrt 5000 5000 94 2500 ? 
  <1-2500>  Input Sustainable Cell Rate(SCR) in Kbps 

7200-1.3(config-if-atm-vc)#vbr-nrt 5000 5000 94 2500 2500 ? 
  <1-65535>  Input Maximum Burst Size(MBS) in Cells

When specifying traffic parameters for a PVC, note that the same vbr-nrt configuration statement does not offer the option of configuring these values since the VC is not performing any signaling.

7200-1.3(config)#interface atm 6/0.2 
7200-1.3(config-subif)#pvc 1/100 

7200-1.3(config-if-atm-vc)#vbr-nrt 5000 5000 ? 
  <1-65535>  Maximum Burst Size(MBS) in Cells 
  <cr> 


7200-1.3(config-if-atm-vc)#vbr-nrt 5000 5000 94 ? 
  <cr> 

Related Information

• ATM Technology Support Pages
• ATM Traffic Management Technology Support
• Technical Support & Documentation - Cisco Systems