ATM Signaling

Document ID: 10503

Contents

Introduction
Prerequisites
      Requirements
      Components Used
      Conventions
Understanding the QSAAL PVC
UNI 3.0, 3.1 and 4.0 Versions
Signaling Messages
      Message Format
      Messages
      Information Elements
Call States
Sample Debug Output
      Choosing a Debug Command
Related Information

Introduction

ATM end systems and network switches use signaling protocols to establish dynamic user connections, known as switched virtual circuits (SVCs). An ATM switch and an ATM end system, such as an ATM-attached router, exchange user to network interface (UNI) signaling messages across UNI links. ATM switches exchange Network to Network Interface (NNI) signaling across NNI links. NNI signaling is part of the Private Network-Network Interface (PNNI) specification, which includes both signaling and routing.

In rare circumstances, SVC connection requests fail or are torn down unexpectedly. This document helps you to troubleshoot SVC connection failures across UNI links.

See also the ATM Signaling and Addressing chapter in the Guide to ATM Technology.

Note: PNNI is outside the scope of this document. Refer to the Configuring ATM Routing and PNNI chapter in the ATM Switch Router Software Configuration Guide for more information.

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.

Understanding the QSAAL PVC

The simplest way to view signaling messages is as control packets that carry important information between ATM switches and end-systems. ATM devices transmit signaling messages over a dedicated PVC configured with a special QSAAL (Quasi Signaling Application Adaptation Layer) ATM adaptation layer encapsulation. By default, the dedicated signaling VC uses virtual path identifier (VPI)=0 and virtual channel identifier (VCI)=5. Cisco Campus ATM switches such as the LightStream 1010 and Catalyst 8500 series create the signaling VC and other well-known (reserved) VCs like ILMI automatically during startup.

Use the show atm vc command to confirm that each interface supports a QSAAL PVC. Note how this VC cross-connects to interface ATM 2/0/0, which identifies the switch's internal management port. Since signaling messages are control messages, they must be sent to and processed by the CPU.

   Switch#show atm vc 
   Interface VPI VCI Type X-Interface X-VPI X-VCI Encap Status 
    atm0/0/0  0 5     PVC  atm2/0/0  0    39   QSAAL   UP 
    atm0/0/0  0 16    PVC  atm2/0/0  0    35   ILMI    UP 
    atm0/0/1  0 5     PVC  atm2/0/0  0    40   QSAAL   UP 
    atm0/0/1  0 16    PVC  atm2/0/0  0    36   ILMI    UP 
    atm0/0/1  4 50    PVC  atm2/0/0  0    230  SNAP    UP

If you need to use a VPI or VCI value other than the default of 0/5 for the QSAAL VC, use the atm manual-well-known-vc command, as shown in the following example:

   Switch(config)# interface atm 0/0/0 
   Switch(config-if)# atm manual-well-known-vc delete 
   Okay to delete well-known VCS for this interface? [no]: y 
   Switch(config-if)# atm pvc 1 35 interface atm0 any-vci encap qsaal

Confirm your settings with the show atm vc interface atm command.

   Switch# show atm vc interface atm 0/0/0 
   Interface VPI VCI Type    X-Interface X-VPI X-VCI Encap Status 
     atm0/0/0 1   35  PVC     atm0        0    150    QSAAL UP

UNI 3.0, 3.1 and 4.0 Versions

Most Cisco ATM interfaces support versions 3.0, 3.1 and 4.0 of the ATM Forum's UNI signaling recommendations. Use the atm uni-version command to manually change the UNI version on an ATM interface.

    3640-2.2(config-if)#atm uni-version ? 
    3.0 UNI Version 3.0 
    3.1 UNI Version 3.1 
    4.0 UNI Version 4.0

By default, ILMI autoconfiguration is enabled on ATM switch interfaces and negotiates the UNI version to the highest version in common between the devices at both ends of the physical link. Avoid manually setting the UNI version unless the attached device does not support autoconfiguration. When manually setting the UNI version, ensure that you also disable autoconfiguration with the no atm auto-configuration command.

Signaling Messages

The ATM Forum UNI specifications are based on the Q.2931 public network signaling protocol developed by the ITU-T. The Q.2931 protocol specifies a call control message format that carries information such as message type (setup, call proceeding, release, and so on) and information elements (IEs), which include addresses and quality of service (QoS).

When troubleshooting signaling issues, the message type and information elements of ATM signaling messages must be understood. We'll look at these in more detail later.

Message Format

The following table describes the fields in a standard Q.2931 message.

Messages

UNI signaling messages have a name which normally appears in documentation in all capital letters. Let's look at the two most common sets of signaling messages.

The UNI protocol also supports three other sets of messages: miscellaneous, global call reference, and point-to-multipoint connections. Since these messages typically are not seen in Cisco ATM debugs, we will not discuss them here.

Information Elements

IEs form the body of a signaling message. Each message contains one or more IEs. The SETUP message contains numerous IEs, which are listed below.

Call States

Networking protocols frequently use state machines that take a device or interface through several stages to normal operation. UNI recommendations specify ATM call control states for both the user side and the network side. Understanding these states is another important piece of information for troubleshooting ATM signaling issues.

The following table lists the call states that may exist on the user side of the UNI link.

The following table lists the call states that may exist on the network side of the UNI link.

Sample Debug Output

Let's now look at some sample debug output in order to understand the information presented so far. In the following output, we can see that ATM interface 12/1/1 receives a SETUP message containing the AAL5 parameters, ATM traffic descriptor, and broadband bearer capability IEs. The user-side state changes from Null (U0) for no call exists to Call Present (U6) to indicate that the interface received an incoming call establishment request but has not yet responded. The reports by the LightStream 1010 of a change in call state are highlighted in bold below.

   LS1010-1# 
   1d03h: ATMSIG(12/1/1:0 0,0 - 0056/00): (vcnum:0) Input event : Rcvd Setup in Null(U0) 
   1d03h: ATMSIG(12/1/1:0 0,0 - 0056/00): (vcnum:0) AAL5 Parameters IE (type 5) 
   1d03h: ATMSIG(12/1/1:0 0,0 - 0056/00): (vcnum:0) 
   ATM Traffic Descriptor IE: Forward PCR01 ID = 58600 Backward PCR01 ID = 58600 Best1 
   1d03h: ATMSIG(12/1/1:0 0,117 - 0056/00): (vcnum:0) 
   Broadband Bearer Capability IE: Bearer class = 0x10 ATC = 0x80 User plane = 0x80 
   1d03h: ATMSIG(12/1/1:0 0,117 - 0056/00): (vcnum:0) Null(U0) -> Call Present(U6)    
   1d03h: SIG->CC: Svc Event Rcvd Setup, 
   State Call Present(U6)atmsig_set_e2e_extqos_def:serv_category 4 extQos 0,
    end2enddelayIe 0 
   1d03h: ATMSIG(12/1/1:0 0,117 - 0056/00): (vcnum:0) sending next port req    
   1d03h: ATMSIG: ROUTING INTERFACE : call ref = 56(0x800038); call end pt ref = 0(0x0); 
    calling port : (12/1/1)    : 0 ; 
   1d03h: Port List : no of ports = 0 
   1d03h: dtl_node : 0:0:00.000000000000000000000000.000000000000.00    
   1d03h: output_port : (0x0)0/0/0:0 
   1d03h: next_hop.type = addr(0x2);
    next_hop addr = 47.000018036680057000000000.00A0B10716B2.0B    
   1d03h: ATMSIG: processing port_rsp message from routing 
   1d03h: ATMSIG: ROUTING INTERFACE : err_code = PNNI_SUCCESS
    (0x1)call ref = 56(0x800038); 
   call end pt ref = 0(0x0); 
    calling port : (12/1/1)    : 0 ; 
   1d03h: Port List : no of ports = 1 
    Port ID 1 : (12/0/0) :    0 
   1d03h: dtl_node : 0:0:00.000000000000000000000000.000000000000.00    
   1d03h: output_port : (0x0)0/0/0:0 
   1d03h: next_hop.type = addr(0x2);
    next_hop addr = 47.000018088770005500000000.00A0B10716B2.0B    
   1d03h: No Received DTLs 
   1d03h: ATMSIG(12/1/1:0 0,117 - 0056/00): (vcnum:0)Call Control nodetype : uni_node 
   1d03h: CC->SIG: Svc Event Req Call Proceeding, State Call Present(U6)    
   1d03h: ATMSIG(12/1/1:0 0,117 - 0056/00): (vcnum:0)
    Input event : Req Call Proceeding in Call Present(U6) 
   1d03h: ATMSIG(12/1/1:0 0,117 - 0056/00): (vcnum:0)
    Output Call Proc msg, Call Present(U6) state 
   1d03h: ATMSIG(12/1/1:0 0,117 - 0056/00): (vcnum:0)
    Call Present(U6) -> Incoming Call Proceeding(U9)

Choosing a Debug Command

Debug commands are a powerful tool to troubleshoot ATM signaling issues. Since using debug commands also can interrupt operation of your Cisco device as well as produce a large amount of output, it is important to choose the correct debug command.

Caution: Before issuing debug commands, please see Important Information on Debug Commands.

   ls1010-2#debug atm sig-? 
   sig-all sig-error sig-events sig-hog  sig-ie 
   sig-mibs sig-mp2p sig-nni sig-packets

Note the following when enabling signaling debug commands:

• debug atm sig-all command does not enable debug atm sig-packet, which must be enabled with a separate configuration command.

       LS1010-1#debug atm sig-all int atm 0/0/0 
       ATM Signaling Events debugging is on 
       ATM Signaling Error debugging is on 
       ATM Signaling IE debugging is on 
       ATM Signaling NNI debugging is on 
       ATM Soft vc/vp Events debugging is on 
       ATM Signaling CALLFAIL debugging is on 
       ATM Signaling Api debugging is on

• Peform the following steps to limit the impact of the debug output on the operation of the ATM switch:

  1. Define a specific interface on which to run the debugs.

  2. Disable logging to the console with the no logging console command.

  3. Enable logging to the buffer, and configure a large buffer size to store the output.

• debug sscop error helps to troubleshoot link up or down events at the logical level. Such events are often due to the service-specific connection-oriented protocol (SSCOP) at the signaling plane that resets all signaling. Use the show atm sig statistics command to see the number of resets.

• Note in particular the location code in the cause IE of a RELEASE message and use the value to isolate your troubleshooting to a problem with an endpoint or a switch in the ATM network.

     Protocol Discriminator:   Q.2931 user-network call control message 
     Call Reference Length:    3 bytes 
      Direction Flag:          Message sent from the side originating call 
      Value:                   0x02c858 
     Message Type:             Release (0x4d) 
      Message AI Flag:         Ignore Action Indicator Field 
      Action Indicator Field:  Not significant 
      Message Length:          7 bytes 
     Information Element Name: Cause (0x08) 
      Coding Standard:         ITU-T standardized 
      Instruction Flag:        Ignore Instruction Field 
      Instruction Field:       Not significant 
      Element Length:          3 bytes 
      Location:                User 
      Cause:                   Invalid information element contents (100) 
      Diagnostics:             Present (listed below) 
      Information Element:     ATM Traffic Descriptor    
  
      Frame 10:    
  
      Protocol Discriminator:   Q.2931 user-network call control message 
       Call Reference Length:   3 bytes 
        Direction Flag:         Message sent to the side originating call 
        Value:                  0x00096e 
       Message Type:            Release (0x4d) 
        Message AI Flag:        Ignore Action Indicator Field 
        Action Indicator Field: Not significant 
        Message Length:         6 bytes 
       Information Element Name: Cause (0x08) 
        Coding Standard:        ITU-T standardized 
        Instruction Flag:       Ignore Instruction Field 
        Instruction Field:      Not significant 
        Element Length:         2 bytes 
        Location:               Private network serving the local user 
        Cause:                  Message not compatible with call state (101)  
    
      Protocol Discriminator:   Q.2931 user-network call control message 
       Call Reference Length:   3 bytes 
        Direction Flag:         Message sent from the side originating call 
        Value:                  0x00096e 
       Message Type:            Release Complete (0x5a) 
        Message AI Flag:        Ignore Action Indicator Field 
        Action Indicator Field: Not significant 
        Message Length:         6 bytes 
       Information Element Name: Cause (0x08) 
        Coding Standard:        ITU-T standardized 
        Instruction Flag:       Ignore Instruction Field 
        Instruction Field:      Not significant 
        Element Length:         2 bytes 
        Location:               User 
        Cause:                  Destination out of order (27)
  

Related Information

• ATM Technology Support Pages
• ATM Signaling and Addressing
• ATM Routing with IISP and PNNI
• Troubleshooting Switch Router ATM Network Connections
• Technical Support - Cisco Systems