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Juniper Networks Default Forwarding Classes


Most M Series routers have only four queues built into the hardware. M120, M320, MX Series, and T Series routers can be configured for up to eight queues. If a classifier does not assign a packet to any other queue (for example, for other than well-known DSCPs that have not been added to the classifier), the packet is assigned by default to the class associated with queue 0 (Q0).

Table 1 shows the four forwarding classes and queues to which Juniper Networks classifiers assign a packet based on the DSCP values in arriving packet headers.

Table 1: Default Forwarding Classes

Forwarding Class Name



queue 0


queue 1


queue 2


queue 3

Each forwarding class has an associated scheduler priority. Only two forwarding classes, best-effort and network-control (Q0 and Q3), are actually referenced in the default scheduler configuration. However, you can manually configure resources for the expedited-forwarding and assured-forwarding classes (Q1 and Q2).

The default scheduler settings are not visible in the output of the show class-of-service command; rather, they are implicit.

Default Scheduler

By default, the best-effort forwarding class (Q0) receives 95 percent of the output link bandwidth and buffer space, and the network-control forwarding class (Q3) receives 5 percent of the output link bandwidth and buffer space. The default drop profile provides tail drop, where the buffer fills and then discards all packets until there is space in the buffer again. There are no schedulers for the expedited-forwarding or assured-forwarding classes because by default no resources are assigned to Q1 and Q2.

All af classes other than af1x are mapped to best-effort, since RFC 2597 prohibits a node from aggregating classes. In effect, mapping to best-effort implies that the node does not support that class.