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JunosE 11.3.x Quality of Service Configuration Guide
Static Oversubscription
Dynamic Oversubscription
Color-Based Thresholding
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18
The router dynamically varies queue lengths for all queues as the real-time demand on
the egress packet memory changes. You can configure limits to prevent the router from
setting queue lengths too low or too high.
The router uses static oversubscription to vary queue thresholds based on the number
of queues currently configured, which is relatively static. Static oversubscription is based
on the assumption that, when a few queues are configured, many of the queues are likely
to be active at the same time. When a large number of queues are configured, fewer
queues are likely to be active at the same time.
When few queues are configured, buffer memory is strictly partitioned between queues
to ensure that buffers are available for all queues. As the number of configured queues
increases, buffer memory is increasingly oversubscribed to allow more buffer sharing.
Reserving buffer space for all queues when many are expected to be idle is unnecessary
and wasteful.
The router uses dynamic oversubscription to vary queue thresholds based on the amount
of egress buffer memory in use. The router divides egress buffer memory into eight regions.
The size of the region depends on the ASIC type. For more information, see "Memory
Requirements for Queue and Buffers" on page 19.
When buffer memory is in low demand, queues are given large amounts of buffer memory.
As the demand for buffer memory increases, queues are given progressively smaller
amounts of buffer memory.
Packets within the router are tagged with a drop precedence:
Committed—Green
Conformed—Yellow
Exceeded—Red
When the queue fills above the exceeded threshold, the router drops red packets, but
still queues yellow and green packets. When the queue fills above the conformed drop
threshold, the router queues only green packets.
NOTE: All color-based thresholds vary in proportion to the dynamic queue
length.
Configuring Queue Profiles to Manage Buffers and Thresholds on page 22
Guidelines for Managing Queue Thresholds on page 19
Guidelines for Managing Buffers on page 20
Copyright © 2010, Juniper Networks, Inc.
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