Guidelines for Configuring QoS over ATM

This section provides general QoS configuration guidelines for ATM line modules. These guidelines are applicable to all JunosE releases.

The SAR scheduler generates VC backpressure as a way to control the flow of packets from the HRR scheduler to the SAR scheduler. The QoS port modes control integration of the two schedulers.

In default integrated mode, each VC queue in the SAR generates backpressure for the ATM VC node in the default traffic class group in the HRR. The backpressure throttles the dequeue rate of the ATM VC node and the nodes and queues stacked above it in the scheduler hierarchy. VC backpressure is disabled in low-latency QoS port mode and low-cdv QoS port mode.

You can configure queues in default integrated mode in the HRR that are immune to VC backpressure so that you can run voice and video applications. Queues and nodes in any named traffic class group are not subject to VC backpressure.

In addition, ATM VP and ATM (port level) queues are not stacked above ATM VC nodes, so queues are not subject to backpressure, regardless of the traffic class group.

Take care not to saturate SAR queues with too much traffic from the HRR, especially when shaping VP tunnels or VCs in the SAR. You can accomplish this in several ways:

Note: These rules apply only to the default integrated mode. VC backpressure is disabled in low-latency or low-cdv modes. You must account for cell tax; to do this, use the qos-shaping-mode cell command for the line module.

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