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Default Schedulers Overview

Each forwarding class has an associated scheduler priority. Only two forwarding classes, best effort and network control (queue 0 and queue 3), are used in the Junos default scheduler 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 3) receives 5 percent. The default drop profile causes the buffer to fill and then discard all packets until it has space.

The expedited-forwarding (queue 1) and assured-forwarding (queue 2) classes have no reserved bandwidth or buffer space because, by default, no schedulers are assigned to those forwarding classes. 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 classes if they receive a larger amount of the offered load than the bandwidth allocated. For more information, see Allocation of Leftover Bandwidth.

The following default scheduler is provided when you install the Junos OS. These settings are not visible in the output of the show class-of-service command; rather, they are implicit.