Guest

Inverse Multiplexing over ATM (IMA)

Inverse Multiplexing Over ATM (IMA) on Cisco 2600 and 3600 Routers

Document ID: 10451

Updated: Jun 05, 2005

   Print

Introduction

Inverse Multiplexing over ATM (IMA) involves inverse multiplexing and de-multiplexing of ATM cells in a cyclical fashion among physical links grouped to form a higher-bandwidth and logical link. The rate of the logical link is approximately the sum of the rate of the physical links in the IMA group. Streams of cells are distributed in a round-robin manner across the multiple T1/E1 links and reassembled at the destination to form the original cell stream. Sequencing is provided using IMA Control Protocol (ICP) cells.

In the transmit direction, the ATM cell stream received from the ATM layer is distributed on a cell by cell basis across the multiple links within the IMA group. At the far-end, the receiving IMA unit reassembles the cells from each link on a cell-by-cell basis and recreates the original ATM cell stream. The image below displays how cell streams are transmitted across multiple interfaces and recombined to form the original cell stream. The receiving interface discards the ICP cells, and the aggregate cell stream is then passed to the ATM layer.

Periodically, the transmit IMA sends special cells that permit reconstruction of the ATM cell stream at the receiving IMA. These ICP cells provide the definition of an IMA frame.

Cell streams are transmitted across multiple interfaces and recombined to form the original stream.

ima-block.gif

Before You Begin

Conventions

For more information on document conventions, see the Cisco Technical Tips Conventions.

Prerequisites

T1 circuits most be terminated "end-to-end" before any IMA implementation can be tested.

Note: In a Lab environment, routers can be connected "back-to-back" via T1 cross-over cables. (PINs 1-4, 2-5).

Components Used

The information in this document is based on the software and hardware versions below.

  • NM-4T1-IMA, NM-8TI-IMA - Provides support for 2600 & 3600 series.

  • AIM-ATM (ATM Advanced Integration Module) - Used with T1/E1 multiflex trunk WAN interface card (VWIC-MFT). Supports up to 4 T1/E1 interfaces and permits bundling with IMA services.

Note: Both modules require any Cisco IOS® "IP Plus" or Service Provider (-p) feature set.

Platform Cisco Minimum IOS® Release
2600 12.0(5)T, 12.0XK, 12.1, 12.1T
2600 series ATM-AIM 12.0(5)T, 12.0XK, 12.1, 12.1T
2600 series ATM-AIM 12.2(2)XA
3600 series ATM-AIM 12.2(2)XB

Cisco's Network Modules (NMs) for the 2600 and 3600 series presently include support for ATM Forum Specification 1.0 (AF-PHY-0086.000), which defines IMA functionality and cell formats (filler and IMA). AIM-ATM modules for 2600 and 3600 series also support IMA 1.1 (AF-PHY-00086.001.) For more information on ATM Forum specifications, please visit the ATM Forum /images/exit.gif web site.

Configure

In this section, you are presented with the information to configure the features described in this document.

Note: To find additional information on the commands used in this document, use the IOS Command Lookup tool

Network Diagram

This document uses the network setup shown in the diagram below.

ima.gif

Configurations

Note: The following information applies to Guilder

Follow these configuration steps in order to configure an IMA group:

  • Follow these configuration steps in order to configure an IMA group:

  • Define the physical level parameters (if required). For example scrambling.

  • Group the E1/T1 interfaces you require (they must be on the same port adapter/network module) with the ima-group command.

The IMA interface has the following syntax:

interface atm x/imay

where x is the slot number and y is the IMA-group number.

Guilder Aguila (Non-IMA)
interface ATM1/0 
 no ip address 
 no atm ilmi-keepalive 
 ima-group 0 

!---Allows ATM1/0 to participate in IMA group 0

 no scrambling-payload 
! 
interface ATM1/1 
 no ip address 
 no atm ilmi-keepalive 
 ima-group 0 

!---Allows ATM1/1 to participate in IMA group 0

 no scrambling-payload 
! 
interface ATM1/IMA0 

!---Creates a virtual IMA interface.

! 
interface ATM1/IMA0.1 point-to-point 
 ip address 10.10.10.1 255.255.255.248 
 no atm ilmi-keepalive 
 pvc 0/33

!---Defines VPI/VCI values

 vbr-nrt 3000 3000 1

!---Configures traffic shaping

interface ATM1/0
 no ip address
 no atm ilmi-keepalive
 no scrambling-payload
!
interface ATM1/0.1 point-to-point
 ip address 10.10.10.2 255.255.255.248
 pvc 0/44
 vbr-nrt 3000 3000 1


!---This example is a non-IMA interface.  
!---Information for configuration of cross- 
!---connect for ATM switches is listed below

 
 

The following are additional considerations regarding this configuration:

Verify

Show Commands

show ima interface atm 1/ima0

guilder#show ima interface atm 1/ima0
Interface ATM1/IMA0 is up
        Group index is 1
        Ne state is operational, failure status is noFailure
        Active links bitmap 0x3
    IMA Group Current Configuration:
        Tx/Rx configured links bitmap 0x3/0x3
        Tx/Rx minimum required links 1/1
        Maximum allowed diff delay is 25ms, Tx frame length 128
        Ne Tx clock mode CTC, configured timing reference link ATM1/0
        Test pattern procedure is disabled
    IMA Group Current Counters (time elapsed 257 seconds):
        0 Ne Failures, 0 Fe Failures, 0 Unavail Secs
    IMA Group Total Counters (last 5 15 minute intervals):
        1 Ne Failures, 1 Fe Failures, 22 Unavail Secs
    IMA link Information:
        Link     Physical Status        NearEnd Rx Status      Test Status 
        ----     ---------------        -----------------      ----------- 
        ATM1/0   up                     active                 disabled    
        ATM1/1   up                     active                 disabled

Description of show ima interface fields:

Field Name Description
Interface ATM1/IMA0 is up Displays status of IMA group.
Group index Value assigned to IMA group at configuration. This information is passed via IMA ICP cells.
Ne state Follows group state machine status. The value displayed provides an indication of the IMA group status. Possible values include: Startup, Start-up-Ack, Config-Aborted, Insufficient-Links, Blocked, Operational.
failure status Provides details related to failure status of the IMA group. Possible values include Not in group, Unusable, Usable, Active, Blocking.
Tx/Rx configured links bitmap 0x3/0x3 Bitmap values are in Hex and represent the links configured in the IMA group and links that are active in a group. The binary values are read right to left to determine physical port value. 0x3 represented in binary is equal to 00000011. Port 0 is to the far right, Port 7 to the left.
Tx/Rx minimum required links 1/1 Minimum number of links required for IMA group to remain up. Value can be changed using the ima active-minimum-links command.
Maximum allowed diff delay A receiving IMA interface should compensate for any delay that is greater than the configured maximum differential delay tolerance. If a link delay exceeds the specified maximum, the link is removed from service. Maximum configurable value is 250ms.
Ne Tx clock mode Configurable values are common and independent. Common clock states that interfaces derive their clock from one single source. Independent implies clocking can be derived from different clock sources.
IMA link Information Describes the physical state of each interface in IMA group.

Certain show atm interface atm 1/ima0

guilder#show atm interface atm 1/ima0
Interface ATM1/IMA0:
AAL enabled:  AAL5 , Maximum VCs: 256, Current VCCs: 3
 
Maximum Transmit Channels: 0
Max. Datagram Size: 4496
PLIM Type: DS1 IMA, Framing is T1 ESF, TX clocking: IMA CTC
304244 input, 309038 output, 0 IN fast, 0 OUT fast, 0 out drop
 Avail bw = 3000 
Config. is ACTIVE

Description of sh atm interface fields:

Field Name Description
Interface ATM1/IMA0 Slot and IMA group number.
Maximum VCs: , Current VCCs: Defines the total amount of VCs configurable per group or interface. (256 for NM and 1024 for AIM.) Current describes the number of VCs presently configured.
PLIM Type: Describes the physical interface type.
Framing is Displays the configured framing method. Configurable values are esfadm or sfadm.
Tx clocking: Describes the configured clocking mode. Configurable values are independent or common.
packets input, packets output Displays the number of packets transmitted and received on the IMA group interface.
Avail bw = Shows usable bandwidth amount. Based on number of T1/E1 interfaces configured in an IMA group.

sh ima interface atm 1/ima0 detailed

guilder#show ima interface atm 1/ima0 detailed 
Interface ATM1/IMA0 is up
        Group index is 1
        Ne state is operational, failure status is noFailure
        Active links bitmap 0x3
    IMA Group Current Configuration:
        Tx/Rx configured links bitmap 0x3/0x3
        Tx/Rx minimum required links 1/1
        Maximum allowed diff delay is 25ms, Tx frame length 128
        Ne Tx clock mode CTC, configured timing reference link ATM1/0
        Test pattern procedure is disabled
    Detailed group Information:
        Tx/Rx Ima_id 0x10/0x0, symmetry symmetricOperation
        Number of Tx/Rx configured links 2/2
        Number of Tx/Rx active links 2/2
        Fe Tx clock mode ctc, Rx frame length 128
        Tx/Rx timing reference link 0/1
        Maximum observed diff delay 0ms, least delayed link 1
        Running seconds 6238
        GTSM last changed 00:00:33 UTC Mon Mar 1 1993
    IMA Group Current Counters (time elapsed 324 seconds):
        0 Ne Failures, 0 Fe Failures, 0 Unavail Secs
    IMA Group Interval(1) Counters:
        0 Ne Failures, 0 Fe Failures, 0 Unavail Secs
    IMA Group Interval(2) Counters:
        0 Ne Failures, 0 Fe Failures, 0 Unavail Secs
    IMA Group Interval(3) Counters:
        0 Ne Failures, 0 Fe Failures, 0 Unavail Secs
    IMA Group Interval(4) Counters:
        0 Ne Failures, 0 Fe Failures, 0 Unavail Secs
    IMA Group Interval(5) Counters:
        1 Ne Failures, 1 Fe Failures, 22 Unavail Secs
    IMA Group Total Counters (last 5 15 minute intervals):
        1 Ne Failures, 1 Fe Failures, 22 Unavail Secs
    Detailed IMA link Information:
 
Interface ATM1/0 is up
        ifIndex 1, Group Index 1, Row Status is active
        Tx/Rx Lid 0/1, relative delay 0ms
        Ne Tx/Rx state active/active
        Fe Tx/Rx state active/active
        Ne Rx failure status is noFailure
        Fe Rx failure status is noFailure
        Rx test pattern 0x40, test procedure disabled
    IMA Link Current Counters (time elapsed 340 seconds):
        0 Ima Violations, 0 Oif Anomalies
        0 Ne Severely Err Secs, 0 Fe Severely Err Secs
        0 Ne Unavail Secs, 0 Fe Unavail Secs
        0 Ne Tx Unusable Secs, 0 Ne Rx Unusable Secs
        0 Fe Tx Unusable Secs, 0 Fe Rx Unusable Secs
        0 Ne Tx Failures, 0 Ne Rx Failures
        0 Fe Tx Failures, 0 Fe Rx Failures
     <omitted>

Description of sh atm interface fields:

Field Name Description
Interface ATM1/IMA0 is up Displays status of IMA group.
Maximum allowed diff delay is A receiving IMA interface should compensate for any delay that is greater than the configured maximum differential delay tolerance. If a link delay exceeds the specified maximum, the link is removed from service. Maximum configurable value is 250ms.
Tx/Rx Ima_id 0x10/0x0 Show transmit and receive IMA IDs.
Fe Tx clock mode ctc Displays far end clocking configuration.
Maximum observed diff delay Displays the actual differential delay.
IMA Group Current Counters Provides information about group failures.
ifIndex , Group Index Interface Index ID and Group Index ID. These values are assigned during interface configuration by the router and are locally significant.
Fe Tx/Rx state is Displays status of far end transmit state as displayed in IMA ICP cells.
Fe Rx failure status is Displays status of far end receive failure as displayed in IMA ICP cells.
IMA Link Current Counters Provides per interface error counts based on intervals.

Troubleshoot

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

Please see Troubleshooting ATM IMA Links on Cisco 2600 and 3600 Routers for a more detailed explanation on how to troubleshoot IMA interfaces on Cisco 2600 and 3600 Routers using the show controllers command.

Common Bugs

The following table lists common bugs for the 2600/3600 series involving the IMA network module:

ID Description
CSCdr39332 T1 IMA differential delay exceeded causes good links to go down. When a single T1 or E1 interface exceeds the differential delay limit on a 3600 series router, other links not affected by the delay could become deactivated. Please see Troubleshooting Bouncing IMA Links for additional information on troubleshooting differential delay on IMA interfaces.
CSCdt64050 NM ATM IMA: vc-per-vp command does not function in some releases. Configuration values are accepted, but are not communicated to the SAR (Segmentation and Reassembly) processor. Please see Understanding the Maximum Number of Active Virtual Circuits on Cisco ATM Router Interfaces for more information on understanding the use of atm vc-per-vp command.
CSCdu49075 IMA Physical Status shows down, but is up after reload. In certain instances, physical status shows down in the output of the show ima interface command. This issue represents a cosmetic bug and has been resolved.

Related Information

Updated: Jun 05, 2005
Document ID: 10451