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Configuring Rate Limits to Protect Lower Queues on IQ2 and Enhanced IQ2 PICs

You can rate-limit strict-high and high priority queues on IQ2 and IQ2E PICs. Without this limiting, traffic in higher priority queues can block the transmission of lower priority packets. Unless limited, higher priority traffic is always sent before lower priority traffic, causing the lower priority queues to “starve,” which in turn leads to timeouts and unnecessary resending of packets.

On the IQ2 and IQ2E PICs 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.


IQ2E PICs exclude the transmit rate of strict-high and high priority queues, thereby allowing low and medium priority queues to be configured up to 100 percent.

To rate-limit queues, include the transmit-rate statement with the rate-limit option at the [edit class-of-service schedulers scheduler-name] hierarchy level:

This example limits the transmit rate of a strict-high expedited-forwarding queue to 1 megabit per second (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:

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-name

  • show class-of-service interface interface-name