IMA provides the capability to send and receive a single high-speed ATM data stream over multiple slower-speed physical links. The originating stream of ATM cells is divided so that complete ATM cells are sent in round-robin order across the set of ATM links.

IMA requires the configuration of a logical ATM interface. The logical ATM interface is called an IMA group, and consists of multiple physical ATM links. VCs are configured under the IMA group interface, and can send data over any or all of the physical ATM links in the group.

When multiple T1 or E1 lines are grouped into an IMA group, the total available bandwidth is the sum of the bandwidth of each line. If one or more of the lines goes down, the total bandwidth available on the IMA group interface is reduced. If a line then come back up, the total available bandwidth increases. These dynamic changes in available total bandwidth impact the bandwidth that is available for any VC configured on the IMA group interface.

When the total available bandwidth on an IMA group interface changes, all of the PVCs configured on that interface are re-created.

If necessary and applicable for a particular PVC based on its QoS class, new values are applied for the following parameters when PVCs are re-created:

• PCR--peak cell rate
• MCR--minimum cell rate
• SCR--sustainable cell rate

The following steps are performed by the Cisco IOS software to determine what value should be assigned to a parameter when a PVC is re-created in response to a change in total available bandwidth:

• A value is calculated for the parameter. The calculation takes into account the configured value for the parameter, the active value for the parameter (if it is different from the configured value), and the change in total available bandwidth.
• The calculated value is compared to the configured value of the parameter and to the maximum available cell rate, and a new value is determined. The new value is applied when the PVC is re-created.

Note	The configured value of a parameters is not overwritten in the configuration file by any new value that is applied in response to dynamic bandwidth changes.

The following sections describe how the new parameter values are determined when a PVC is re-created for supported QoS classes:

• UBR PVCs
• ABR PVCs
• VBR-NRT PVCs

The IMA Dynamic Bandwidth feature allows Cisco IOS software to make dynamic adjustments to VC bandwidth in response to changes in the overall IMA interface bandwidth.

Perform this task to enable the IMA Dynamic Bandwidth feature.

Note	Incorrect QoS parameters may be applied to PVCs if the IMA Dynamic Bandwidth feature is disabled after a change in total bandwidth, and then reenabled after another change in total bandwidth.

1.    enable

2.    configure terminal

3.    interface atm slot /ima group-number

4.    atm bandwidth dynamic

The following example creates IMA group 1, enables automatic bandwidth management, and assigns a physical ATM interface to the IMA group:

interface atm3/ima 1
 atm bandwidth dynamic
!
interface atm0/1
 ima-group 1