Understanding Cos Schedulers; Default Schedulers - Juniper JUNOS OS 10.4 - FOR EX REV 1 Manual

Understanding CoS Schedulers

Default Schedulers

Copyright © 2010, Juniper Networks, Inc.
You use schedulers to define the properties of output queues. These properties include
the amount of interface bandwidth assigned to the queue, the size of the memory buffer
allocated for storing packets, the priority of the queue, and the drop profiles associated
with the queue.
You associate the schedulers with forwarding classes by means of scheduler maps. You
can then associate each scheduler map with an interface, thereby configuring the queues,
packet schedulers, and tail drop processes that operate according to this mapping.
Default Schedulers on page 3387
Transmission Rate on page 3388
Scheduler Buffer Size on page 3388
Priority Scheduling on page 3388
Scheduler Drop-Profile Maps on page 3389
Scheduler Maps on page 3389
Each forwarding class has an associated scheduler priority. Only two forwarding classes,
best-effort (queue0) and network-control (queue7) are used in the default configuration.
NOTE: On Juniper Networks EX8200 Ethernet Switches three forwarding
classes—best-effort (queue0), multicast best-effort (queue2), and
network-control (queue7)—are used in the default configuration.
By default, the best-effort forwarding class (queue 0) receives 95 percent of the
bandwidth and buffer space for the output link, and the network-control forwarding class
(queue 7) receives 5 percent. The default drop profile causes the buffer to fill completely
and then to discard all incoming packets until it has free space.
NOTE: On EX8200 switches, by default, the best-effort forwarding class
(queue 0) receives 75 percent of the bandwidth, the multicast best-effort
forwarding class (queue 2) receives 20 percent of the bandwidth and buffer
space for the output link, and the network-control forwarding class (queue
7) receives 5 percent.
The expedited-forwarding and assured-forwarding classes have no scheduler because
no resources are assigned to queue 5 and queue 1, by default. However, you can manually
configure resources for the expedited-forwarding and assured-forwarding classes.
Also by default, each queue can exceed the assigned bandwidth if additional bandwidth
is available from other queues. When a forwarding class does not fully use the allocated
transmission bandwidth, the remaining bandwidth can be used by other forwarding
Chapter 113: Class of Service (CoS)—Overview
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