Inverse Multiplexing over ATM (IMA)

Document ID: 10463

Contents

Introduction
Prerequisites
      Requirements
      Conventions
To Understand Show Ima Output
Status of the State Machine
IMA Group Current Configuration
IMA MIB Information
IMA Group Counters
Physical Link Counters
IMA Link Counters
To Understand Show Controllers Output
Known Issues
      Line Protocol Down on Physical Interface
      Active-Links-Minimum for Bandwidth
Supported MIBs
Related Information

Introduction

Before you can ping across an inverse-multiplexing-over-ATM (IMA) interface, you need to ensure that both the IMA logical interface and the physical interfaces that make up that IMA logical interface are up and active. In some cases, these interfaces do not show as up/up in the output of the show interface command. There are two key commands to help determine the error condition:

• show ima interface atm{mod}/ima{group#} detail - Provides IMA-specific information on both the IMA group interface and the composing physical interfaces.

• show controllers atm {mod/port} (7200 Series) or show controller vip <slot#> tech (7500 Series) - Provides detailed information on the physical interfaces and the ATM segmentation and reassembly (SAR).

This document explains the values you can see in the output of these commands. Much of the information used in this document is defined in Appendix A of AF-PHY-0086.001, which is the specification for the IMA of the ATM Forum. Click here to download the IMA and other approved specifications.

Prerequisites

Requirements

There are no specific requirements for this document.

Conventions

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

To Understand Show Ima Output

The show ima interface atm command displays four sets of information that can help troubleshoot IMA link problems:

• Near end status, far end status, and group status

• IMA group current configuration

• IMA MIB information

• IMA group counters

   Router#show ima interface atm 1/ima0 detail 
     ATM1/ima0 is up 
           ImaGroupState:NearEnd = operational, FarEnd = operational 
           ImaGroupFailureStatus  =    noFailure 
   IMA Group Current Configuration:    
           ImaGroupMinNumTxLinks = 2    ImaGroupMinNumRxLinks = 2 
           ImaGroupDiffDelayMax  = 25   ImaGroupNeTxClkMode   = common(ctc) 
           ImaGroupFrameLength = 128    ImaTestProcStatus     = disabled 
           ImaGroupTestLink = 0         ImaGroupTestPattern   = 0xFF 
   IMA MIB Information: 
           ImaGroupSymmetry           = symmetricOperation 
           ImaGroupFeTxClkMode        = common(ctc) 
           ImaGroupRxFrameLength      = 128 
           ImaGroupTxTimingRefLink    = 0     ImaGroupRxTimingRefLink = 0 
           ImaGroupTxImaId            = 0     ImaGroupRxImaId         = 0 
           ImaGroupNumTxCfgLinks      = 2     ImaGroupNumRxCfgLinks   = 2 
           ImaGroupNumTxActLinks      = 2     ImaGroupNumRxActLinks   = 2 
           ImaGroupLeastDelayLink     = 1     ImaGroupDiffDelayMaxObs = 0 
   IMA group counters: 
           ImaGroupNeNumFailures      = 78    ImaGroupFeNumFailures   = 68 
           ImaGroupUnAvailSecs        = 441453 ImaGroupRunningSecs =  445036 

These sections explain each of the subsections in more detail.

Status of the State Machine

Both physical ATM T1 interfaces and the IMA virtual group interface follow a state machine, in which the interface moves through several states before it becomes active. IMA group interfaces start in the Not Configured state and then enter the Start-up state.

Note: The state machine is defined separately for both the near end the far end.

    ATM1/ima0 is up 
           ImaGroupState:NearEnd = operational, FarEnd = operational 
           ImaGroupFailureStatus  = noFailure 

IMA Group Current Configuration

This output provides the equivalent SNMP object ID of a configuration setting for your ATM IMA interface. Each parameter uses a default setting.

   IMA Group Current Configuration:    
           ImaGroupMinNumTxLinks = 2     ImaGroupMinNumRxLinks = 2 
           ImaGroupDiffDelayMax  = 25    ImaGroupNeTxClkMode   = common(ctc) 
           ImaGroupFrameLength = 128     ImaTestProcStatus = disabled 
           ImaGroupTestLink  = 0         ImaGroupTestPattern = 0xFF 

IMA MIB Information

   IMA MIB Information: 
         ImaGroupSymmetry         = symmetricOperation 
         ImaGroupFeTxClkMode      = common(ctc) 
         ImaGroupRxFrameLength    = 128 
         ImaGroupTxTimingRefLink  = 0      ImaGroupRxTimingRefLink = 0 
         ImaGroupTxImaId          = 0      ImaGroupRxImaId         = 0 
         ImaGroupNumTxCfgLinks    = 2      ImaGroupNumRxCfgLinks   = 2 
         ImaGroupNumTxActLinks    = 2      ImaGroupNumRxActLinks   = 2 
         ImaGroupLeastDelayLink   = 1      ImaGroupDiffDelayMaxObs = 0 

IMA Group Counters

   IMA group counters: 
         ImaGroupNeNumFailures    = 78    ImaGroupFeNumFailures   = 68 
         ImaGroupUnAvailSecs      = 441453  ImaGroupRunningSecs = 445036 

Physical Link Counters

An IMA interface builds an IMA link table that contains status information about a link, which is part of an IMA group. Each link is assigned an interface index (IfIndex), which network management stations use to poll for statistics or set configuration parameters.

    ATM1/0 is up 
           ImaLinkRowStatus = LinkRowStatusUnknown    
           ImaLinkIfIndex   =    0       ImaLinkGroupIndex= 0 
           ImaLinkState: 
                      NeTx = active 
                      NeRx = active 
                      FeTx = active 
                      FeRx = active 
           ImaLinkFailureStatus: 
                      NeRx = noFailure 
                      FeRx = noFailure 
           ImaLinkTxLid              = 0      ImaLinkRxLid = 0 
           ImaLinkRxTestPattern      = 65     ImaLinkTestProcStatus  = disabled 
           ImaLinkRelDelay           = 0 

An IMA interface follows a state machine, in which the interface moves through several states before it becomes active. Special cells, called IMA control protocol (ICP) cells, carry the state information between the two ends. This table explains the possible states:

When the near end reaches a Usable receive state, it moves into an Active receive state only if the far end (FE) reports a Usable transmit state. Similarly, when the near end reaches a Usable transmit state, it moves into an Active transmit state only if the far end reports an Active receive state.

Note: IMA defines separate states for the transmit and receive directions at each end, and each end can report a different state.

IMA Link Counters

   IMA Link counters : 
         ImaLinkImaViolations   = 1 
         ImaLinkNeSevErroredSec = 41      ImaLinkFeSevErroredSec = 34 
         ImaLinkNeUnavailSec    = 441505 ImaLinkFeUnAvailSec     = 28 
         ImaLinkNeTxUnusableSec = 2       ImaLinkNeRxUnUsableSec = 441542 
         ImaLinkFeTxUnusableSec = 74      ImaLinkFeRxUnusableSec = 57 
         ImaLinkNeTxNumFailures = 0       ImaLinkNeRxNumFailures = 15 
         ImaLinkFeTxNumFailures = 4       ImaLinkFeRxNumFailures = 3  

In order to understand the counters in the table requires you to understand these terms:

• Unavailable second (UAS) - A second in which the interface was not available because the router detected 10 contiguous severely errored seconds or a condition that led to a failure.

• Severely errored second (SES) - A second with one or more out of frame defects or a detected alarm indication signal (AIS) defect.

• Unusable - An ATM physical interface enters the UnusableFailed state when it is configured to be a member of an IMA group, but it cannot be used because of one of these fault conditions:

  • Link Defects - Interface experiences loss of signal, loss of frame, alarm indication signal, or loss of cell delineation.

  • Loss of IMA Frame (LIF) - Interface experiences an out of IMA frame (OIF) or loss of synchronization state for at least two IMA frames. When this fault condition is active, the IMA sublayer replaces all received data cells with filler cells.

  • Loss of Delay Synchronization (LODS) - Interface has detected that the timing delay between this link and other links in the IMA group has exceeded the configured maximum value. This value ranges from 25 to 250 milliseconds. Normally, the IMA protocol adjusts for differences in delay so that all links in a group actively carry network traffic. Otherwise, an ATM interface that exceeds the differential timing delay max value is removed from the IMA group. Refer to Troubleshooting Bouncing IMA Links for more information.

To Understand Show Controllers Output

The PA-A3-8T1IMA uses the ATMizer II segmentation and reassembly (SAR) chip. The name of the SAR is displayed in the output of show controllers atm.

   7200-2#show controller atm 3/0 
   Interface ATM3/0 is up 
   Hardware is ENHANCED ATM PA - DS3 (45Mbps) 
   Lane client mac address is 0030.7b1e.9054 
   Framer is PMC PM7345 S/UNI-PDH, SAR is LSI ATMIZER II 
   Firmware rev: G119, Framer rev: 1, ATMIZER II rev: 3 
     idb=0x61499630, ds=0x6149E9C0, vc=0x614BE940 
     slot 3, unit 2, subunit 0, fci_type 0x005B, ticks 73495 
     400 rx buffers: size=512, encap=64, trailer=28, magic=4 
   [snip]

This simple diagram illustrates the path that bits take when they are received from the physical wire at the PA-A3-8T1IMA.

Note: There are various locations inside the system where cells and reassembled packets are stored.

DS1 -> PMC PM734 S/UNI-PDH -> ATMizer II SAR Rx cell buffer -> PCI bus -> RX host buffers

The show controllers interface atm{mod#}/ima{group#} command displays several important values. These values are highlighted in bold:

    Curr Stats: 
     rx_cell_lost=0, rx_no_buffer=0, rx_crc_10=0    
     rx_cell_len=0, rx_no_vcd=0, rx_cell_throttle=0, tx_aci_err=0 
      

Known Issues

Line Protocol Down on Physical Interface

Before Cisco IOS^® Software Release 12.0(11)S and 12.1(3)E, ATM physical interfaces configured as a member of an IMA group were able to have a "line protocol is down" status even though the IMA interface was up/up and passed traffic without any problems. This state was able to occur on the Cisco 7200 Router Series because the ATM interfaces are the raw links, and all protocol-related configuration is done on the virtual IMA interface. Cisco Bug ID CSCdr22203 changed this default behavior on the 7200 Router. On the Cisco 7500 and 3600 Series, the line protocol status on the ATM physical interfaces is brought up when the links are operational.

Active-Links-Minimum for Bandwidth

Originally, the bandwidth of an IMA group interface was limited to the minimum number of active IMA links needed to keep the IMA interface up. The command to define this value is ima active-links-minimum. For example, if four physical ATM interfaces are configured as members of IMA group zero and the ima active-links-minimum value is set to one, the bandwidth is equal to one T1 or 1.5Mbps, not 6 Mbps.

Cisco Bug ID CSCdr12395 changes this behavior. Now the PA-A3-8T1IMA adapter uses the bandwidth of all up/up ATM physical interfaces configured as IMA group members.

Cisco bug IDs CSCdt67354 and CSCdv67523 are subsequent enhancement requests to update the IMA group VC bandwidth when an interface is added or removed from the IMA group, shut/no shut, or bounces due to a link failure or change at the remote end. The changes implemented in CSCdr12395 configure the IMA group bandwidth to the total bandwidth of its member links only when the IMA group comes up. Changes to the IMA group after the initial up status are not reflected.

Supported MIBs

The IMA port adapter supports the IMA MIB and the DS1 MIB (Request for Comments [RFC] 1406) when used in either the 7200 Series or 7500 Series. (We recommend that you use Cisco IOS 12.2 or 12.2T for the latest functionality.) Refer to the Cisco MIBs page.

In this sample output, we use the snmpwalk command from a UNIX workstation to poll the IMA MIB variables on a router with an IP address of 10.1.1.1.

# snmpwalk -m all 10.1.1.1 public imaGroupNumber    
   enterprises.atmForum.atmForumNetworkManagement.atmfIma.atmfImaMib.atmfImaMib   
   Objects.imaGroupNumber.0 = 1 

# snmpwalk -m all -O s 10.1.1.1 public atmfImaMibObjects    
   imaGroupNumber.0 = 1 
   imaGroupRowStatus.126 = active(1) 
   imaGroupIfIndex.126 = 126 
   imaGroupNeState.126 = operational(9) 
   imaGroupFeState.126 = operational(9) 
   imaGroupFailureStatus.126 = noFailure(1)    
   imaGroupSymmetry.126 = symmetricOperation(1)    
   imaGroupMinNumTxLinks.126 = 1 
   imaGroupMinNumRxLinks.126 = 1 
   imaGroupNeTxClkMode.126 = ctc(1) 
   [output omitted] 
   imaLinkInvalidIntervals.2 = 0 
   imaLinkInvalidIntervals.3 = 0 
   imaLinkTimeElapsed.1 = 0
   imaLinkTimeElapsed.2 = 0 
   imaLinkTimeElapsed.3 = 0

Related Information

• Understanding ATM over E1 Framing Formats on IMA Interfaces
• Inverse Multiplexing over ATM on Cisco 7X00 Routers and ATM Switches
• Tools and Utilities - Cisco Systems
• Technical Support & Documentation - Cisco Systems