Relative Strict-Priority Scheduling Overview; Figure 11: Sample Strict-Priority Scheduling Hierarchy - Juniper JUNOSE 11.1.X - QUALITY OF SERVICE CONFIGURATION GUIDE 3-21-2010 Configuration Manual

JUNOSe 11.1.x Quality of Service Configuration Guide

Figure 11: Sample Strict-Priority Scheduling Hierarchy

One strict priority traffic-class group is called the auto-strict-priority group. The
scheduler nodes and queues in the auto-strict-priority group receive strict-priority
scheduling. If multiple queues above the strict-priority node have packets, the HRR
algorithm selects which strict-priority queue is scheduled next.
NOTE: If you configured traffic shaping through traffic shape profiles in JUNOSe
releases before Release 4.0, traffic shaping is replaced with the rate-shaping feature,
which is configured when you configure a scheduler profile.

Relative Strict-Priority Scheduling Overview

Relative strict-priority scheduling provides strict-priority scheduling within a shaped
aggregate rate. For example, it allows you to provide 1 Mbps of aggregate bandwidth
to a subscriber, with up to 500 Kbps of the bandwidth for low-latency traffic. If there
is no strict-priority traffic, the low-latency traffic can use up to the full aggregate rate
of 1 Mbps.
Relative strict priority differs from true strict priority in that it can implement the
aggregate shaping rate for both strict and nonstrict traffic. With true strict priority,
you can shape the nonstrict or the strict traffic separately, but you cannot shape the
aggregate to a single rate.
The best application of relative strict priority is on Ethernet, where you can shape
the aggregate for each VLAN to a specified rate, and provision a strict and nonstrict
queue for each VLAN above the shaped VLAN node.
To use relative strict priority, you configure strict-priority queues above the VC or
VLAN scheduler node, thereby providing for strict-priority scheduling of the queues
within the VC or VLAN. You configure relative strict priority without using QoS
traffic-class groups, which causes strict-priority queues to appear in the same scheduler
hierarchy as the nonstrict queues.
Relative strict priority provides low latency only if you undersubscribe the port by
shaping all VCs on the port so that the sum of the shaping rates is less than the port
60
Strict-Priority and Relative Strict-Priority Scheduling Overview