Note	After configuring the parameters for an ATM PVC, you must exit interface-ATM-VC configuration mode in order to create the PVC and enable the settings.

To create a PVC on the ATM interface and enter interface-ATM-VC configuration mode, use the following command beginning in interface configuration mode:

Router(config-if)# pvc [name] vpi / vci [ilmi | qsaal | smds]

The ATM interface supports a static mapping scheme that identifies the network address of remote hosts or routers. This section describes how to map a PVC to an address, which is a required task for configuring a PVC.

Note	If you enable or disable broadcasting directly on a PVC using the protocol command, this configuration will take precedence over any direct configuration using the broadcast command.

See examples of PVC configurations in the section "ATM Configuration Examples" at the end of this chapter.

To map a protocol address to a PVC, use the following command in interface-ATM-VC configuration mode:

Router(config-if-atm-vc)# protocol protocol protocol-address [[no] broadcast]

To configure the ATM adaptation layer (AAL) and encapsulation type, use the following command beginning in interface-ATM-VC configuration mode:

Router(config-if-atm-vc)# encapsulation aal5 encap

The supported traffic parameters are part of the following service categories: Available Bit Rate (ABR), Unspecified Bit Rate (UBR), UBR+, Variable Bit Rate Non Real-Time (VBR-NRT), and real-time Variable Bit Rate (VBR). Only one of these categories can be specified per PVC connection, so if a new one is entered, it will replace the existing one.

Note	The commands in this section are not supported on the ATM port adapter (PA-A1 series). The ABR service class is only supported on the ATM-CES port adapter for PVCs. The 1-port ATM-25 network module only supports UBR.

The -pcr and -mcr arguments are the peak cell rate and minimum cell rate, respectively. The -scr and -mbs arguments are the sustainable cell rate and maximum burst size, respectively.

For an example of how to configure an ABR PVC, refer to the section "Example Configuring an ABR PVC" at the end of this chapter.

For a description of how to configure traffic parameters in a VC class and apply the VC class to an ATM interface or subinterface, refer to the section "Configuring VC Classes."

To configure PVC traffic parameters, use one of the following commands beginning in interface-ATM-VC configuration mode:

Router(config-if-atm-vc)# abr output-pcr output-mcr

Router(config-if-atm-vc)# ubr output-pcr

Router(config-if-atm-vc)# ubr+ output-pcr output-mcr

Router(config-if-atm-vc)# vbr-nrt  output-pcr output-scr output-mbs

Router(config-if-atm-vc)# vbr-rt peak-rate average-rate burst

For ABR VCs, you can optionally configure the amount that the cell transmission rate increases or decreases in response to flow control information from the network or destination. For an example of configuring an ABR PVC, see the section "Example Configuring an ABR PVC" later in this chapter.

To configure this option, use the following command in interface-ATM-VC configuration mode:

Router(config-if-atm-vc)# atm abr rate-factor [rate-increase-factor] [rate-decrease-factor]

1.    Do one of the following:

• Router(config)# interface atm slot /0
• Router(config)# interface atm slot / port-adapter /0

2.    Router(config-if)# pvc [name] 0/16 ilmi

3.    Router(config-if-atm-vc)# exit

4.    Router(config-if)# atm ilmi-pvc-discovery [subinterface]

5.    Router(config-if)# exit

6.    Do one of the following:

• Router(config)# interface atm slot /0[. subinterface-number {multipoint | point-to-point}]
• Router(config)# interface atm slot / port-adapter /0[. subinterface-number {multipoint | point-to-point}]

7.    Router(config-subif)# ip address ip-address mask

Router(config)# interface atm number

Router(config)# interface atm number[. subinterface-number {multipoint | point-to-point}]

1.    Do one of the following:


• Router(config)# interface atm slot /0[. subinterface-number {multipoint | point-to-point}]
• Router(config)# interface atm slot / port-adapter /0[. subinterface-number {multipoint | point-to-point}]

2.    pvc [name] vpi / vci


3.    encapsulation aal5snap


4.    inarp minutes

Router(config)# interface atm number[. subinterface-number {multipoint | point-to-point}]

1.    Router(config-if-atm-vc)# oam-pvc [manage] frequency

2.    Router(config-if-atm-vc)# oam retry up-count down-count retry-frequency

To send duplicate broadcast packets for all protocols configured on a PVC, use the following command in interface-ATM-VC configuration mode:

Note	If you enable or disable broadcasting directly on a PVC using the protocol command, this configuration will take precedence over any direct configuration using the broadcast command.

Router(config-if-atm-vc)# broadcast

By creating a VC class, you can preconfigure a set of default parameters that you may apply to a PVC. To create a VC class, refer to the section "Configuring VC Classes" later in this chapter.

Once you have created a VC class, use the following command in interface-ATM-VC configuration mode to apply the VC class to a PVC:

Router(config-if-atm-vc)# class-vc  vc-class-name

You can configure the PVC to provide failure notification by sending a trap when a PVC on an ATM interface fails or leaves the UP operational state.

In an SVC environment, you must configure a PVC for communication with the Integrated Local Management Interface (ILMI) so that the router can receive SNMP traps and new network prefixes. The recommended vpi and vci values for the ILMI PVC are 0 and 16, respectively.

Note	This ILMI PVC can be set up only on an ATM main interface, not on ATM subinterfaces.

ILMI address registration for receipt of SNMP traps and new network prefixes is enabled by default. The ILMI keepalive function is disabled by default; when enabled, the default interval between keepalives is 3 seconds.

For an example of configuring ILMI, see the section "Example Configuring Communication with the ILMI" in the "ATM Configuration Examples" section at the end of this chapter.

To configure ILMI communication and optionally enable the ILMI keep alive funciton, use the following command in interface configuration mode:

Router(config-if)# pvc [name] 0/16 ilmi

Router(config-if)# atm ilmi-keepalive [seconds]

Unlike X.25 service, which uses in-band signaling (connection establishment done on the same circuit as data transfer), ATM uses out-of-band signaling. One dedicated PVC exists between the router and the ATM switch, over which all SVC call establishment and call termination requests flow. After the call is established, data transfer occurs over the SVC, from router to router. The signaling that accomplishes the call setup and teardown is called Layer 3 signaling or the Q.2931 protocol .

For out-of-band signaling, a signaling PVC must be configured before any SVCs can be set up. The figure below illustrates that a signaling PVC from the source router to the ATM switch is used to set up two SVCs. This is a fully meshed network; workstations A, B, and C all can communicate with each other.

Figure 2. One or More SVCs Require a Signaling PVC

Note	This signaling PVC can be set up only on an ATM main interface, not on ATM subinterfaces.

The VPI and VCI values must be configured consistently with the local switch. The standard value for VPI and VCI are 0 and 5, respectively.

See the section "Example SVCs in a Fully Meshed Network" at the end of this chapter for a sample ATM signaling configuration.

To configure the signaling PVC for all SVC connections, use the following command in interface configuration mode:

Router(config-if)# pvc [name] vpi / vci qsaal

Every ATM interface involved with signaling must be configured with a network service access point (NSAP) address. The NSAP address is the ATM address of the interface and must be unique across the network.

Once you specify a name for an SVC, you can reenter interface-ATM-VC configuration mode by simply entering the svc namecommand; you can remove an SVC configuration by entering the no svc namecommand.

To create an SVC, use the following commands beginning in interface configuration mode.

Note	Cisco IOS software does not support creation of SVCs on a point-to-point subinterface.

1.    Router(config-if)# svc [name] nsap address

2.    Router(config-if-atm-vc)# encapsulation aal5 encap

3.    Router(config-if-atm-vc)# protocol protocol protocol-address [[no] broadcast]

Normally, when ILMI link autodetermination is enabled on the interface and is successful, the router takes the user-network interface (UNI) version returned by ILMI. If the ILMI link autodetermination process is unsuccessful or ILMI is disabled, the UNI version defaults to 3.0. You can override this default by using the atm uni-version command. The no form of the command sets the UNI version to the one returned by ILMI if ILMI is enabled and the link autodetermination is successful. Otherwise, the UNI version will revert to 3.0. To override the ATM UNI version used by the router, use the following command in interface configuration mode:

Router(config-if)# atm uni-version  version-number

You can specify an interval of inactivity after which any idle SVC on an interface is torn down. This timeout interval might help control costs and free router memory and other resources for other uses.

In addition to configuring the interval of inactivity, you can optionally specify the minimum-ratein kilobits per second (kbps). This is the minimum traffic rate required on an ATM SVC to maintain the connection.

To change the idle timeout interval, use the following command in interface-ATM-VC configuration mode:

Router(config-if-atm-vc)# idle-timeout seconds [minimum-rate]

1.    Do one of the following:

• Router(config)# interface atm slot /0
• Router(config)# interface atm slot / port-adapter /0

2.    Router(config-if)# pvc [name] 0/5 qsaal

3.    Router(config-if-atm-vc)# exit

4.    Router(config-if-atm-vc)# pvc [name] 0/16 ilmi

5.    Do one of the following:

• Router(config-if)# atm nsap-address nsap-address
• Router(config-if)# atm esi-address esi . selector

6.    Router(config-if)# svc [name] nsap address

7.    Router(config-if-atm-vc)# protocol protocol protocol-address broadcast

8.    Router(config-if-atm-vc)# exit

9.    Router(config-if)# atm multipoint-signalling

10.    Router(config-if)# atm multipoint-interval interval

Router(config)# interface atm number

and

Router(config-if-atm-vc)# exit

This task is documented in the "Configuring IP Multicast Routing" chapter of the Cisco IOS IP Configuration Guide .

The tasks in this section are optional and advanced. The ATM signaling software can specify to the ATM interface on the router and the switch a limit on how much traffic the source router will be sending. It provides this information in the form of traffic parameters. (These parameters have default values.) The ATM switch in turn sends these values as requested by the source to the ATM destination node. If the destination cannot provide such capacity levels, the call may fail. (For Cisco router series behavior, see the per-interface atm sig-traffic-shaping strict command in the Cisco IOS Wide-Area Networking Command Reference.) There is a single attempt to match traffic values.

The supported traffic parameters are part of the following service categories: Unspecified bit rate (UBR), UBR+, and variable bit rate nonreal-time (VBR-NRT). Only one of these categories can be specified per SVC connection so if a new one is entered, it will replace the existing one. The commands used to specify the service category and traffic values are identical to those used when you create a PVC.

Note	The commands in this section are not supported on the ATM port adapter (PA-A1 series). The 1-port ATM-25 network module only supports UBR.

The -pcr and -mcr arguments are the peak cell rate and minimum cell rate, respectively. The -scr and -mbs arguments are the sustainable cell rate and maximum burst size, respectively.

For an example of configuring traffic parameters on an SVC, see the section "Example Configuring SVC Traffic Parameters" at the end of this chapter.

For a description of how to configure traffic parameters in a VC class and apply the VC class to an ATM interface or subinterface, refer to the section "Configuring VC Classes."

To configure traffic parameters on an SVC, use the following commands beginning in global configuration mode:

1.    Do one of the following:

• Router(config)# interface atm slot /0[. subinterface-number {multipoint | point-to-point}]
• Router(config)# interface atm slot / port-adapter /0[. subinterface-number {multipoint | point-to-point}]

2.    Router(config-if)# svc [name] nsap address

3.    Router(config-if-atm-vc)# protocol protocol protocol-address [[no] broadcast]

4.    Do one of the following:

• Router(config-if-atm-vc)# ubr output-pcr [input-pcr]
• Router(config-if-atm-vc)# ubr+ output-pcr output-mcr [input-pcr] [input-mcr]

5.    Router(config-if-atm-vc)# exit

Router(config)# interface atm number[. subinterface-number {multipoint | point-to-point}]

Router(config-if-atm-vc)# vbr-nrt  output-pcr output-scr output-mbs [input-pcr] [input-scr] [input-mbs]

You can configure strict traffic shaping on an ATM interface to specify that an SVC be established using only signaled traffic parameters. If such shaping cannot be provided, the SVC is released.

If you do not configure strict traffic shaping on the router ATM interface, an attempt is made to establish an SVC with traffic shaping for the transmit cell flow per the signaled traffic parameters. If such shaping cannot be provided, the SVC is installed with default shaping parameters; that is, it behaves as though a PVC were created without specifying traffic parameters.

To specify that an SVC be established on an ATM interface using only signaled traffic parameters, use the following command in interface configuration mode:

Router(config-if)# atm sig-traffic-shaping strict

You can optionally configure the SVC to generate end-to-end F5 OAM loopback cells to verify connectivity on the virtual circuit. The remote end must respond by echoing back such cells. If OAM response cells are missed (indicating the lack of connectivity), the SVC is torn down. For more information, refer to the "Configuring OAM Management for PVCs and SVCs" section later in this chapter.

Note	Generally, ATM signaling manages ATM SVCs. Configuring the oam-svc command on an SVC verifies the inband integrity of the SVC.

To configure transmission of end-to-end F5 OAM loopback cells on an SVC, use the following commands in interface-ATM-VC configuration mode:

1.    Router(config-if-atm-vc)# oam-svc [manage] frequency

2.    Router(config-if-atm-vc)# oam retry up-count down-count retry-frequency

To send duplicate broadcast packets or send a single broadcast packet using multipoint signaling for all protocols configured on an SVC, use the following command in interface-ATM-VC configuration mode:

Router(config-if-atm-vc)# broadcast

By creating a VC class, you can preconfigure a set of default parameters that you may apply to an SVC. To create a VC class, refer to the section "Configuring VC Classes" later in this chapter.

Once you have created a VC class, use the following command in interface-ATM-VC configuration mode to apply the VC class to an SVC:

Router(config-if-atm-vc)# class-vc  vc-class-name

The Service-Specific Connection-Oriented Protocol (SSCOP) resides in the service-specific convergence sublayer (SSCS) of the ATM adaptation layer (AAL). SSCOP is used to transfer variable-length service data units (SDUs) between users of SSCOP. SSCOP provides for the recovery of lost or corrupted SDUs.

Note	The tasks in this section customize the SSCOP feature to a particular network or environment and are optional. The features have default values and are valid in most installations. Before customizing these features, you should have a good understanding of SSCOP and the network involved.

You can disconnect an idle SVC by using the following command in EXEC mode:

Router # atmsig close atm  slot /0 vcd

To create a VC class, use the following command in global configuration mode:

For examples of creating VC classes, see the section "Example Creating a VC Class" at the end of this chapter.

Router(config)# vc-class atm  name

After you create a VC class and enter vc-class configuration mode, configure VC parameters using one or more of the following commands:

• abr

• broadcast

• encapsulation aal5

• idle-timeout

• ilmi manage

• inarp

• oam-pvc

• oam retry

• oam-svc

• protocol

• ubr

• ubr+

• vbr-nrt

Refer to the sections "Configuring PVCs" and "Configuring PVC Trap Support" for descriptions of how to configure these commands for PVCs and SVCs.

If an SVC command (for example, idle-timeout or oam-svc) is configured in a VC class, but the VC class is applied on a PVC, the SVC command is ignored. This is also true if a PVC command is applied to an SVC.

For examples of creating VC classes, see the section "Example Creating a VC Class" at the end of this chapter.

1.    Do one of the following:

• Router(config-if)# pvc [name] vpi / vci
• Router(config-if)# svc [name] nsap address

2.    Router(config-if-atm-vc)# class-vc vc-class-name

1.    Do one of the following:

• Router(config)# interface atm slot /0[. subinterface-number {multipoint | point-to-point}]
• Router(config)# interface atm slot / port-adapter /0[. subinterface-number {multipoint | point-to-point}]

2.    Router(config-if)# class-int vc-class-name

Router(config)# interface atm number[. subinterface-number {multipoint | point-to-point}]

1.    Do one of the following:

• Router(config)# interface atm slot /0[. subinterface-number {multipoint | point-to-point}]
• Router(config)# interface atm slot / port-adapter /0[. subinterface-number{multipoint | point-to-point}]

2.    Router(config-if)# pvc [name] 0/16 ilmi

3.    Do one of the following:

• Router(config)# interface atm slot /0. subinterface-number multipoint
• Router(config)# interface atm slot / port-adapter /0. subinterface-number multipoint

4.    Router(config-if)# pvc [name] vpi / vci

5.    Router(config-if-atm-vc)# ilmi manage

Router(config)# interface atm number[. subinterface-number {multipoint | point-to-point}]

Router(config)# interface atm number . subinterface-number multipoint

OAM management may be enabled for both PVCs and SVCs. To configure OAM management, perform the tasks in one or both of the following sections:

The ATM ARP mechanism is applicable to networks that use SVCs. It requires a network administrator to configure only the device’s own ATM address and that of a single ATM ARP server into each client device. When the client makes a connection to the ATM ARP server, the server sends ATM Inverse ARP requests to learn the IP network address and ATM address of the client on the network. It uses the addresses to resolve future ATM ARP requests from clients. Static configuration of the server is not required or needed.

In Cisco’s implementation, the ATM ARP client tries to maintain a connection to the ATM ARP server. The ATM ARP server can tear down the connection, but the client attempts once each minute to bring the connection back up. No error messages are generated for a failed connection, but the client will not route packets until the ATM ARP server is connected and translates IP network addresses.

For each packet with an unknown IP address, the client sends an ATM ARP request to the server. Until that address is resolved, any IP packet routed to the ATM interface will cause the client to send another ATM ARP request. When the ARP server responds, the client opens a connection to the new destination so that any additional packets can be routed to it.

Cisco routers may be configured as ATM ARP clients to work with any ATM ARP server conforming to RFC 1577. Alternatively, one of the Cisco routers in a logical IP subnet (LIS) may be configured to act as the ATM ARP server itself. In this case, it automatically acts as a client as well. To configure classical IP and ARP in an SVC environment, perform the tasks in one of the following sections:

The ATM Inverse ARP mechanism is applicable to networks that use PVCs, where connections are established, but the network addresses of the remote ends are not known. A server function is not used in this mode of operation.

By default, Inverse ARP datagrams will be sent on this virtual circuit every 15 minutes. To adjust the Inverse ARP time period, use the inarp minutes command in interface-ATM-VC configuration mode.

Note	The ATM ARP mechanism works with IP only. The Inverse ATM ARP mechanism works with IP and IPX only. For all other protocols, the destination address must be specified.

In a PVC environment, the ATM Inverse ARP mechanism is enabled by default for IP and IPX when you use the following commands beginning in global configuration mode:

1.    Do one of the following:

• Router(config)# interface atm slot /0
• Router(config)# interface atm slot / port-adapter /0

2.    Router(config-if)# ip address address mask

3.    Router(config-if)# pvc [name] vpi / vci

4.    Router(config-if-atm-vc)# no shutdown

5.   

Router(config)# interface atm number

A rate queue defines the speed at which individual virtual circuits will transmit data to the remote end. You can configure permanent rate queues, allow the software to set up dynamic rate queues, or perform some combination of the two. The software dynamically creates rate queues when you create a VC with a peak rate that does not match any user-configured rate queue. The software dynamically creates all rate queues if you have not configured any.

Note	You can only configure the rate queue for the AIP and NPM.

Each interface has a default maximum packet size or maximum transmission unit (MTU) size. For ATM interfaces, this number defaults to 4470 bytes. The maximum is 9188 bytes for the AIP and NPM, 17969 for the ATM port adapter, and 17998 for the ATM-CES port adapter. The MTU can be set on a per-sub-interface basis as long as the interface MTU is as large or larger than the largest subinterface MTU.

For ATM interfaces, the MTU displays the following behavior:

• A new subinterface inherits the MTU from the main interface.

• The MTU is inherited from the main interface when there is a default setting on the subinterface MTU.

• A user configured MTU is nv-gened, even if the MTU is the same as/default MTU or the sub MTU of the main interface.

• If you have not explicitly specified the MTU, after a reload, by default, the subinterface MTU inherits the main interface.

• A change in the main interface MTU does not trigger a change in the subinterface MTUs. However, subsequently created subinterfaces inherit the same main interface MTU.

To set the maximum MTU size, use the following command in interface configuration mode:

Router(config-if)# mtu bytes

The default SONET PLIM is STS-3C. The default for DS3 is C-Bit ADM framing; the default for E3 is G.751 with PLCP framing.

To set the SONET PLIM to STM-1 or to set the PLIM framing for E3 or DS3, use one of the following commands in interface configuration mode:

Router(config-if)# atm sonet stm-1

Router(config-if)# atm framing [cbitadm | cbitplcp | m23adm | m23plcp]

Router(config-if)# atm framing [g751adm | g832 adm | g751plcp]

Use these commands to set the loopback mode:

Router(config-if)# loopback

Router(config-if)# loopback line

The exception queue is used for reporting ATM events, such as CRC errors.

Note	This command is supported only on the AIP.

By default, it holds 32 entries; the range is 8 to 256. It is unlikely that you will need to configure the exception queue length; if you do, use the following command in interface configuration mode:

Router(config-if)# atm exception-queue  number

The atm max-channels command, available if you are using the ATM-CES port adapter, can be used to divide the available number (fixed) of transmit descriptors across the configured number of transmit channels. Typically, you think of a one-to-one association between a transmit channel and a VC; however, the ATM-CES port adapter supports types of VCs other than data VCs (for example CES VCs). Also, the ATM-CES port adapter can multiplex one or more VCs over a single virtual path (VP) that is shaped, and the VP only requires a single transmit channel. Therefore, the term transmit channel is used rather than virtual circuit.

The maximum burst of packets that are allowed per VC is limited by the number of transmit descriptors allocated per VC. Because the total number of transmit descriptors available is limited by the available SRAM space, configuration of the number of transmit channels for the interface determines the number of transmit descriptors for each transmit channel. Hence, the burst size for each transmit channel is determined by the atm max-channels command. For example, for 64 (default) numbers of transmit channels for the interface, 255 transmit descriptors are associated per transmit channel and for 512 numbers of transmit channels for the interface, 31 transmit descriptors are associated per transmit channel.

Note	This command is available only on the ATM-CES port adapter.

To configure the maximum number of transmit channels for the interface, use the following command in interface configuration mode:

Router(config-if)# atm max-channels  number

By default, the ATM interface allows the maximum of 2048 virtual circuits. However, you can configure a lower number, thereby limiting the number of virtual circuits on which your ATM interface allows segmentation and reassembly to occur. Limiting the number of virtual circuits does not affect the VPI-VCI pair of each virtual circuit.

Note	This command is not supported on the ATM-CES port adapter or the NPM.

To set the maximum number of virtual circuits supported (including PVCs and SVCs), use the following command in interface configuration mode:

Router(config-if)# atm maxvc  number

The raw queue is used for raw ATM cells, which include operation, administration, and maintenance (OAM) and Interim Local Management Interface (ILMI) cells. ILMI is a means of passing information to the router, including information about virtual connections and addresses. The raw-queue size is in the range of 8 to 256 cells; the default is 32 cells.

Note	This command is supported only on the AIP.

To set the raw-queue size, use the following command in interface configuration mode:

Router(config-if)# atm rawq-size  number

The number of receive buffers determines the maximum number of reassemblies that your ATM interface can perform simultaneously. The number of buffers defaults to 256, although it can be in the range from 0 to 512.

The number of transmit buffers determines the maximum number of fragmentations that your ATM interface can perform simultaneously. The number of buffers defaults to 256, although it can be in the range from 0 to 512.

Note	The commands in this section are not supported on the ATM-CES port adapter or NPM.

To set the number of receive and transmit buffers, use the following commands in interface configuration mode:

Router(config-if)# atm rxbuff  number

Router(config-if)# atm txbuff  number

By default, the ATM interface supports 1024 VCIs per VPI. Depending on what ATM interface card or port adapter you are using, this value can be any power of 2 in the range of 16 to 8192. (See the atm vc-per-vp command in the Cisco IOS Wide-Area Networking Command Reference for the exact values that apply to your configuration.) This value controls the memory allocation on your ATM interface that deals with the VCI table. It defines only the maximum number of VCIs to support per VPI.

To set the maximum number of VCIs to support per VPI and limit the highest VCI accordingly, use the following command in interface configuration mode:

Router(config-if)# atm vc-per-vp  number

By default, your ATM interface expects the ATM switch to provide transmit clocking. To specify that the ATM interface generates the transmit clock internally for SONET and E3 PLIM operation, use the following command in interface configuration mode:

Router(config-if)# atm clock internal

1.    Router(config-if)# atm mid-per-vc maximum

2.    Router(config-if)# pvc [name] vpi / vci smds

3.    Router(config-if-atm-vc)# mid midlow midhigh

The virtual path filter allows you to specify which VPI or range of VPIs will be used for AAL3/4 processing. The default value of the’s virtual path filter register is 0x7B. To set the virtual path filter register, use the following command in interface configuration mode:

Router(config-if)# atm vp-filter  hexvalue

In the transmit direction, IMA takes cells from the ATM layer and sends them in sequential distribution over the individual links that make up a logical link group called an IMA group (links can also be used individually instead of being a member of a group). The IMA group performance is approximately the sum of the links, although some overhead is required for ATM control cells. At the receiving end, the cells are recombined to form the original cell stream and are passed up to the ATM layer.

Filler cells are used to ensure a steady stream on the receiving side. IMA control protocol (ICP) cells control the operation of the inverse multiplexing function. With a frame length of 128, one of every 128 cells on each link is an ICP cell. The inverse multiplexing operation is transparent to the ATM layer protocols; therefore, the ATM layer can operate normally as if only a single physical interface were being used.

The figure below illustrates inverse multiplexing and demultiplexing with four bundled links, providing 6.144 Mbps of raw bandwidth for T1s and 7.68 Mbps of raw bandwidth for E1 for packet traffic. The transmit side, where cells are distributed across the links, is referred to as Tx , and the receive side, where cells are recombined, is called Rx .

Figure 4. Inverse Multiplexing and Demultiplexing

ATM networks were designed to handle the demanding performance needs of voice, video, and data at broadband speeds of 34 Mbps and above. However, the high cost and spotty availability of long-distance broadband links limits broadband ATM WANs, preventing many organizations from taking advantage of the power of ATM. In response to these issues, the ATM Forum defined lower-speed ATM interface options for T1 and E1. However, this was not a complete solution because a single T1 or E1 link often does not provide enough bandwidth to support either traffic among different router and switch locations or heavy end-user demand.

For this reason, many organizations find themselves caught between the bandwidth limitations of a narrowband T1 or E1 line and the much higher costs of moving to broadband links. In response to this dilemma, the ATM Forum, with Cisco as an active member, defined Inverse Multiplexing for ATM (IMA). Using Cisco routers to provide ATM access gives branch offices and enterprises an affordable LAN-to-ATM interface.

For a list of ATM features that are supported on Cisco routers when you use the Multiport T1/E1 ATM Network Module with Inverse Multiplexing over ATM or the Multiport T1/E1 ATM Port Adapter with Inverse Multiplexing over ATM, see the "Cisco ATM Features" section of the "Wide-Area Networking Overview" chapter in this book.

The following sections describe the configuration and verification tasks required to set up ATM IMA groups. You can also configure ATM links individually, but these sections include only the steps for configuring IMA groups. To configure and verify IMA groups on an ATM interface, complete the tasks in the following sections. Each task is identified as optional or required.

For examples of IMA configuration, see the section "Examples Inverse Multiplexing over ATM" at the end of this chapter.

To configure the ATM interface for IMA operation, perform the tasks in one of the following two sections:

To verify that the ATM interface is configured correctly for IMA operation, perform the steps in one of the following sections:

As shown in the previous section, the ima-group command configures links on an ATM interface as IMA group members. When IMA groups have been set up in this way, you can configure settings for each group. To configure IMA groups and settings for each group, perform the tasks in one of the following two sections: