Permanent Virtual Circuits (PVC) and Switched Virtual Circuits (SVC)

Introduction

This sample configuration solves a communication problem that occurs on a permanent virtual circuit (PVC). The network connectivity is lost (no traffic), but the PVC remains up on the end devices. The result is that routing entries that point to the PVC remain in the routing table, and therefore packets are lost. The solution is to use the Operation, Administration, and Maintenance (OAM) feature to detect such failures and allow the PVC to go down if there is a disruption along its path. This document shows two router configurations with and without OAM.

Note: This document provides an introductory sample configuration for OAM. Refer to Troubleshooting PVC Failures When Using OAM Cells and PVC Management for more detailed information on OAM.

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 Release 12.0 and later. Limited support exists in Cisco IOS Software Release 11.1CC. However, an upgrade to Cisco IOS Software Release 12.0 or later is recommended.

• A card such as a 7200+PA-A1. This is the card that is used in testing this sample configuration with Cisco IOS® Software Release 12.0. However, the card type is not important in this sample configuration. If you use a PA-A3, for example, the show atm pvc command gives more information, but none of the information is relevant to this feature.

The information in this document was created from the devices in a specific lab environment. All of the devices used in this document started with a cleared (default) configuration. If your network is live, make sure that you understand the potential impact of any command.

Conventions

Refer to the Cisco Technical Tips Conventions for more information on document conventions.

Configure

Network Diagram

This document uses this network setup:

For this document, these statements apply:

• 1/116 is the Virtual Path Identifier/Virtual Channel Identifier (VPI/VCI) assigned for the PVC on both sides of the ATM switch.

• 1/116 is switched to 1/116 by the ATM switch.

• Logical Link Control (LLC) encapsulation is used for the configuration example. This is configured using aal5snap.

Configurations

Without OAM

Without OAM, the router configurations on both sides of the switch look like the output shown in these tables:

interface ATM1/0.116 multipoint 
  ip address 10.0.0.2 255.0.0.0 
  pvc 1/116 
   protocol ip 10.0.0.1 broadcast 
   encapsulation aal5snap

interface ATM2/0/0.116 multipoint 
  ip address 10.0.0.1 255.0.0.0 
  pvc 1/116  
   protocol ip 10.0.0.2 broadcast 
   encapsulation aal5snap

If the PVC on Guilder goes down, it remains up on Bernard, as shown in these tables:

Guilder(config)#interface ATM1/0.116 multipoint 

 Guilder(config-subif)#shutdown

 Guilder#show interface atm 1/0.116
    ATM1/0.116 is administratively down, line protocol is down
    [snip]

Bernard#show interface atm 2/0/0.116  
   ATM2/0/0.116 is up, line protocol is up 
   [snip] 
 Bernard#show atm vc interface atm 2/0/0.116
                VCD /                                     Peak Avg/Min Burst
 Interface      Name         VPI   VCI  Type   Encaps     Kbps   Kbps  Cells Sts
 2/0/0.116      4              1   116   PVC    SNAP     149760                UP

 Bernard#show ip route

  
 Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP
        D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area 
        N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
        E1 - OSPF external type 1, E2 - OSPF external type 2, E - EGP
        i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, ia - IS-IS inter area
        * - candidate default, U - per-user static route, o - ODR
  
 Gateway of last resort is not set
  
 R    100.0.0.0/8 [120/1] via 10.0.0.2, 00:00:07, ATM2/0/0.116
 C    10.0.0.0/8 is directly connected, ATM2/0/0.116C

 C    40.0.0.0/8 is directly connected, BVI2
      11.0.0.0/22 is subnetted, 1 subnets
 C       11.200.8.0 is directly connected, Ethernet0/0/0

With OAM

These tables show what the router configurations look like with OAM and PVC management enabled:

interface ATM1/0.116 multipoint
  ip address 10.0.0.2 255.0.0.0 
  pvc 1/116 
   protocol ip 10.0.0.1 broadcast 
   oam-pvc manage  
   encapsulation aal5snap

interface ATM2/0/0.116 multipoint 
  ip address 10.0.0.1 255.0.0.0 
  pvc 1/116  
   protocol ip 10.0.0.2 broadcast 
    oam-pvc manage
   encapsulation aal5snap

If the PVC on Guilder goes down, the PVC goes down on Bernard, as shown in these tables:

Guilder#configure terminal
 Enter configuration commands, one per line.  End with CNTL/Z.
 Guilder(config)#interface atm 1/0.116
 
 Guilder(config-subif)#shutdown

 Guilder#show interfaces atm 1/0.116 
 ATM1/0.116 is administratively down, line protocol is down
 [snip]
 Guilder#show atm vc
             VCD /                                      Peak  Avg/Min Burst
 Interface   Name       VPI   VCI  Type   Encaps   SC   Kbps   Kbps   Cells  Sts
 1/0.116    3            1   116   PVC    SNAP     UBR  155000              INAC

Bernard#show atm vc
                VCD /                                     Peak Avg/Min Burst
 Interface      Name         VPI   VCI  Type   Encaps     Kbps   Kbps  Cells Sts
 2/0/0.116      4              1   116   PVC    SNAP     155000              DOWN

 Bernard#show ip route
 Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP
        D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area 
        N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
        E1 - OSPF external type 1, E2 - OSPF external type 2, E - EGP
        i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, * - candidate default
        U - per-user static route, o - ODR
        T - traffic engineered route
  
 Gateway of last resort is not set
  
 C    40.0.0.0/8 is directly connected, BVI2
      11.0.0.0/22 is subnetted, 1 subnets
 C       11.200.8.0 is directly connected, Ethernet0/0/0
      
 Bernard#show interfaces atm 2/0/0.116 
 ATM2/0/0.116 is down, line protocol is down 
 [snip] 

 Bernard#show atm pvc 1/116 
 ATM2/0/0.116: VCD: 4, VPI: 1, VCI: 116
 UBR, PeakRate: 155000
 AAL5-LLC/SNAP, etype:0x0, Flags: 0xC20, VCmode: 0x0
 OAM frequency: 10 second(s), OAM retry frequency: 1 second(s)
 OAM up retry count: 3, OAM down retry count: 5
 OAM Loopback status: OAM Sent
 OAM VC state: Not Verified
 ILMI VC state: Not Managed
 VC is managed by OAM.
 InARP frequency: 15 minutes(s)
 InPkts: 39, OutPkts: 53, InBytes: 3504, OutBytes: 5636
 InPRoc: 36, OutPRoc: 21, Broadcasts: 33
 InFast: 0, OutFast: 0, InAS: 3, OutAS: 0
 OAM cells received: 345
 F5 InEndloop: 194, F5 InSegloop: 0, F5 InAIS: 151, F5 InRDI: 0
 F4 InEndloop: 0, F4 InSegloop: 0, F4 InAIS: 0, F4 InRDI: 0
 OAM cells sent: 477
 F5 OutEndloop: 326, F5 OutSegloop: 0, F5 OutRDI: 151
 F4 OutEndloop: 0, F4 OutSegloop: 0, F4 OutRDI: 0
 OAM cell drops: 0
 Status: DOWN, State: NOT_VERIFIED

Verify

There is currently no verification procedure available for this configuration.

Troubleshoot

This section provides information you can use to troubleshoot your configuration.

Troubleshooting Commands

The Output Interpreter Tool (registered customers only) (OIT) supports certain show commands. Use the OIT to view an analysis of show command output.

Note: Refer to Important Information on Debug Commands before you issue debug commands.

• debug atm oam—Displays the OAM cells and general information about OAM.

• show interfaces atm—Displays information about the ATM interface.

• show atm pvc—Displays all ATM permanent virtual circuits (PVCs) and traffic information. Works only in Cisco IOS Software Release 11.3T and later.

• show atm vc—Displays all ATM virtual circuits and traffic information. Provides less information than show atm pvc, but is available in software releases prior to Cisco IOS Software Release 11.3T.

• show ip route—Displays the IP routing table.

For OAM and PVC management, you can also use the oam retry <count 1> <count 2> <count 3> command:

• <count 1> is the OAM retry count before declaring a VC is up.

• <count 2> is the OAM retry count before a VC is declared down.

• <count 3> is the OAM retry polling frequency.

Summary

• With OAM disabled:

  • The status of a PVC depends on the status of the corresponding physical interface. Subsequently, PVCs underneath interfaces that are shutdown or in a failure status display as down, while PVCs underneath an interface that is connected show up.

• With oam-pvc manage enabled:

  • The status of a PVC depends on the receipt of OAM Loopback echo-replies.

  • PVC is declared down if:

    • Five (default) consecutive F5 Loopback Cells are not received back with a Loopback indicator of 1.

    • Alarm Indication Signal (AIS) or Remote Defect Indicator (RDI) cells are received that indicate a failure along the VC path.

• With oam-pvc <0-600> configured without the manage keyword:

  • The router sends OAM F5 Loopback Cells but does not disable the subinterface PVC if they are not received with a Loopback Indicator of 1. This is a pertinent tool for troubleshooting in a live network environment.

Related Information

• Operation, Administration, and Maintenance (OAM) Enhancement
• Technical Support & Documentation - Cisco Systems