Configuring Low-Latency Mode for Per-Port Queuing on ATM Interfaces

In low-latency mode, the SAR scheduler backpressures the HRR scheduler per physical port; each physical port buffers only a few kilobytes.

When you configure low-latency mode:

• VC backpressure is disabled.
• Port backpressure is enabled as aggressive.
• SAR scheduler performs minimal buffering.
• HRR scheduler is dominant.

This procedure creates the low-latency mode configuration shown in Figure 1.

Figure 1: Low-Latency Mode

To configure low-latency mode with a strict-priority queue and a best-effort queue:

1. Configure the traffic class.
   host1(config)#traffic-class strict host1(config-traffic-class)#exit
2. Set the traffic class in the traffic-class group.
   host1(config)#traffic-class-group strict host1(config-traffic-class-group)#traffic-class strict host1(config-traffic-class-group)#exit
3. Define the scheduler profile for the traffic-class group.
   host1(config)#scheduler-profile strict host1(config-scheduler-profile)#strict-priority host1(config-scheduler-profile)#exit
4. Configure the QoS profile with two ATM VC queues.
   host1(config)#qos-profile low-latency-q-p host1(config-qos-profile)#atm-vc node host1(config-qos-profile)#atm-vc queue traffic-class best-effort host1(config-qos-profile)#atm group strict scheduler-profile strict host1(config-qos-profile)#atm-vc queue traffic-class strict host1(config-qos-profile)#exit
5. From the desired port, set the QoS port mode to low latency.
   host1(config)#interface atm 2/0 host1(config-if)#qos-mode-port low-latency host1(config-if)#qos-profile low-latency-q-p

   Tip: For ATM interfaces on ERX7xx models, ERX14xx models, and the ERX310 router, you must specify port 0.

   The qos-mode-port command:

   • Excludes non-UBR ATM QoS services on any VC on the ATM module; for example, PCR, nrtVBR, and CBR
   • Cannot be used if shaping is currently configured on the SAR scheduler
   • Cannot be used with ATM VP tunnels with nonzero rates; however, can be used with tunnels with rates of zero (shapeless tunnels).