Configuring Rate Limits on Enhanced Queuing DPCs

You can rate-limit the strict-high and high queues on the Enhanced Queuing DPC. 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” and cause timeouts and unnecessarily resent packets.

On the Enhanced Queuing DPC 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. For more information about configuring CoS on IQ2 PICs, see CoS on Enhanced IQ2 PICs Overview.

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:

[edit class-of-service schedulers scheduler-name]transmit-rate rate rate-limit;

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:

[edit class-of-service]schedulers {scheduler-1 {transmit-rate 1m rate-limit; # This establishes the limitpriority strict-high;}}scheduler-maps {scheduler-map-1 {forwarding-class expedited-forwarding scheduler scheduler-1;}}[edit interfaces]s0-2/2/0 {per-unit-scheduler;encapsulation frame-relay;unit 0 {dlci 1;}}[edit class-of-service]interfaces {so-2/2/0 {unit 0 {scheduler-map scheduler-map-1;shaping-rate 2m;}}}

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