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# 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 45.

Figure 45: 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 ERX-7xx models, ERX-14xx models, and the ERX-310 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).

#### Related Topics

- Per-Packet Queuing on the SAR Scheduler Overview
- Guidelines for Configuring QoS over ATM
- interface atm command
- qos-mode-port command
- qos-profile command

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