Juniper JUNOSE 11.1.X - QUALITY OF SERVICE CONFIGURATION GUIDE 3-21-2010, Configuration Manual

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Static and Hierarchical Assured Rate Overview
You can configure the effective weight of the scheduler node or queue by configuring
a static assured rate or a hierarchical assured rate (HAR). The JUNOSe hierarchical
assured rate (HAR) feature provides a more powerful and efficient method of
configuring assured rates than static assured rates.
When you use static assured rates, a queue is guaranteed to receive its assured rate
only when its parent node is configured with an assured rate that equals the sum of
all its child assured rates. Therefore, to ensure that a queue receives its specified
assured rate, you must frequently recalculate the assured rates on all parent nodes
in the queue’s hierarchy. This recalculation is necessary because of the number of
scheduler nodes and queues that may be dynamically created or deleted through
applications such as bandwidth-on-demand. Eventually, this complicated manual
recalculation process becomes unreasonable and virtually impossible.
HAR replaces the manual recalculation process by directing the router to dynamically
calculate the assured rate for a scheduler node based on the sum of the assured rates
of all its child nodes and queues. For example, you might use HAR to increase the
effective weight of an ATM-VC scheduler node when a video queue is created, and
to later restore the effective rate of the node when the video queue is deleted.
HAR is applicable only to level 1 and level 2 scheduler nodes, and is not applicable
to queues or ports. When you configure HAR, the changes take place immediately.
When you disable HAR, the scheduler node’s previous weight is restored.
Figure 10 on page 55 shows an application of HAR for VC nodes. In the example,
VCs, which are configured for HAR, are stacked over virtual path (VP) nodes. The VP
nodes are in turn stacked over an OC-3 ATM port. Each VC has a best-effort data
queue, which currently has an assured rate of 20 Kbps. The VCs share equal portions
of their parent VP's bandwidth. However, when the video queue is added to VC2,
HAR enables VC2's share of the VP bandwidth to increase in proportion to the 1-Mbps
video queue that was created. The bandwidth of sibling VC nodes, which have only
a data queue, is decreased in equal proportions.
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Static and Hierarchical Assured Rate Overview
JUNOSe 11.1.x Quality of Service Configuration Guide