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Configuring Rate-Limiting Policers for High Priority Low-Latency Queues on IQE PICs

You can rate-limit the strict-high and high queues on the IQE PIC. Without this limiting, traffic that requires low latency (delay) such as voice can block the transmission of medium-priority and low-priority packets. Unless limited, high and strict-high traffic is always sent before lower priority traffic, causing the lower priority queues to “starve” and cause timeouts and unnecessarily resent packets.

On the IQE PIC you can rate-limit queues before the packets are queued for output. All packets exceeding the configured rate limit are dropped, so care is required when establishing this limit. This model is also supported on IQ2 PICs and is the only way to perform egress policing on IQE PICs. This feature introduces no new configuration statements.

Although intended for low-latency traffic classes such as voice, the configuration allows any queue to be rate-limited. However, the configuration requires the rate-limited queue to have either a high or strict-high priority.


You can configure a low-latency static policer for only one rate-limited queue per scheduler map. You can configure up to 1024 low-latency static policers.

This example limits the transmit rate of a strict-high expedited-forwarding queue to 1 Mbps. The scheduler and scheduler map are defined, and then applied to the traffic at the [edit interfaces] and [edit class-of-service] hierarchy levels:

  1. Define the scheduler:
    1. Specify a name for the scheduler to create it.

    2. Specify the transmit rate.

    3. Specify the priority of the scheduler.

  2. Define the scheduler map:
    1. Specify a name for the scheduler map to create it.

    2. Map the EF forwarding class to the scheduler.

  3. Configure the physical interface.

    This example uses Frame Relay encapsulation and enables per-unit scheduling, which enables you to apply scheduling to the Frame Relay DLCI.

    1. Specify the physical interface of the interface.

    2. Enable the association of scheduler map names with logical interfaces.

    3. Specify the encapsulation type.

  4. Configure the logical interface and specify the Frame Relay DLCI.
    1. Specify the logical interface number.

    2. Specify the data-link connection identifier (DLCI).

  5. Apply the scheduler map to the logical interface:
    1. Specify the physical and logical interface to which you want to apply the scheduler map.

    2. Specify the name of the scheduler map you created.

    3. Specify the amount of bandwidth to be allocated for the logical interface.

  6. You can issue the following operational mode commands to verify your configuration (the first shows the rate limit in effect):
    • show class-of-service scheduler-map scheduler-map scheduler-map-1

    • show class-of-service interface so-2/0/0