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Default Scheduler Settings

Each forwarding class has an associated scheduler priority. Only two forwarding classes, best-effort (ID 0, queue 0) and network-control (ID 3, queue 7), are used in the Junos OS default scheduler configuration.

By default, the best-effort forwarding class (queue 0) receives 95 percent, and the network-control (queue 7) receives 5 percent of the bandwidth and buffer space for the output link. The default drop profile causes the buffer to fill and then discard all packets until it again has space.

The expedited-forwarding and assured-forwarding classes have no schedulers, because by default no resources are assigned to queue 5 (ID 1) and queue 1 (ID 2). However, you can manually configure resources for the expedited-forwarding and the assured-forwarding classes.


The ID refers to the forwarding class ID assigned by the COSD daemon. COSD assigns a forwarding class ID to every forwarding class. The ID is unique to a forwarding-class and is used as a unique identifier in any internal communication with the PFE. PFE side software knows nothing about forwarding-class names but only IDs. So, there is one-to-one mapping from forwarding class name to ID.

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 offered load than the bandwidth allocated. If you do not want a queue to use any leftover bandwidth, you must configure it for strict allocation.

The device uses the following default scheduler settings. You can configure these settings.