Document ID: 10401
Updated: Nov 15, 2007
Contents
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:
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Central site—7 MB PCR=SCR, connected to LS1010 port 0/0/0
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Remote site—3.5 MB PCR=SCR, connected to LS1010 port 1/0/0
| LS1010 Configuration |
|---|
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.
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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
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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.
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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
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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
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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.
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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>
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Refer to Cisco Technical Tips Conventions for information on conventions used in this document.