Schedulers And Relative Strict Priority; Relative Strict Priority On Atm Modules; Figure 13: Relative Strict-Priority Configuration - Juniper JUNOSE SOFTWARE FOR E SERIES 11.3.X - QUALITY OF SERVICE CONFIGURATION GUIDE 2010-09-22 Configuration Manual

JunosE 11.3.x Quality of Service Configuration Guide

Schedulers and Relative Strict Priority

Figure 13: Relative Strict-Priority Configuration

Relative Strict Priority on ATM Modules

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nonstrict packet is the propagation delay of a single large packet at the port rate. For a
1500 byte frame at OC3 rate, that latency is less than 100 microseconds.
Because the strict and nonstrict packets for a VC are scheduled in separate round robins,
the scheduler cannot enforce an aggregate rate for both of them.
In the relative strict-priority configuration in Figure 13 on page 60, the scheduler provides
relative strict-priority scheduling relative to the VC. If the port is not oversubscribed, the
VC round robin does not cause significant latency.
This configuration provides a latency bound for the relative strict-priority queues. The
worst-case latency caused by a nonstrict packet is the propagation delay of a single
large packet at the VC rate. For a 1500 byte frame at a 2 Mbps rate, that delay is about
6 milliseconds.
This configuration provides for shaping the aggregate of nonstrict and relative strict
packets to a single rate, and it is consistent with the traditional ATM model. It does not
scale as well as true strict priority, because the nonstrict and relative strict traffic together
must not oversubscribe the port rate.
You can use relative strict priority on any type of E Series line module; however, on ATM
line modules you have an alternative. On ATM line modules you can configure true
strict-priority queues in the HRR scheduler and shape the aggregate for the VC in the
SAR scheduler. VC backpressure affects only the nonstrict traffic for the VC. For this type
of configuration, you should shape the relative strict traffic for each VC in the HRR
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