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forwarding-classes (CoS)

SRX Series

M320, MX Series, T Series, EX Series, PTX Series

Hierarchy Level


Command used to associate forwarding classes with class names and queues with queue numbers.

All traffic traversing the SRX Series device is passed to an SPC to have service processing applied. Junos OS provides a configuration option to enable packets with specific Differentiated Services (DiffServ) code points (DSCP) precedence bits to enter a high-priority queue or a medium-priority queue or low-priority queue on the SPC. The Services Processing Unit (SPU) draws packets from the highest priority queue first, then from the medium priority queue, last from the low priority queue. The processing of queue is weighted-based not strict-priority-based. This feature can reduce overall latency for real-time traffic, such as voice traffic.

Initially, the spu-priority queue options were "high" and "low". Then, these options (depending on the devices) were expanded to "high", "medium-high", "medium-low", and "low". The two middle options ("medium-high" and "medium-low") have now been deprecated (again, depending on the devices) and replaced with "medium". So, the available options for spu-priority queue are "high", "medium", and "low".

We recommend that the high-priority queue be selected for real-time and high-value traffic. The other options would be selected based on user judgement on the value or sensitivity of the traffic.

For M320, MX Series, T Series routers and EX Series switches only, you can configure fabric priority queuing by including the priority statement. For Enhanced IQ PICs, you can include the policing-priority option.


The priority and policing-priority options are not supported on PTX Series Packet Transport Routers.


  • class class-name—Displays the forwarding class name assigned to the internal queue number.


    This option is supported only on SRX5400, SRX5600, and SRX5800.


    AppQoS forwarding classes must be different from those defined for interface-based rewriters.

  • priority—Fabric priority value:

    • high—Forwarding class’ fabric queuing has high priority.

    • low—Forwarding class’ fabric queuing has low priority.

    The default priority is low.

  • queue queue-number—Specify the internal queue number to which a forwarding class is assigned.

  • spu-priority—Services Processing Unit (SPU) priority queue, high, medium, or low. The default spu-priority is low.


    The spu-priority option is supported only on SRX5000 line devices.

Required Privilege Level

interface—To view this statement in the configuration.

interface-control—To add this statement to the configuration.

Release Information

Statement introduced before Junos OS Release 7.4.

policing-priority option introduced in Junos OS Release 9.5.

Statement updated in Junos OS Release 11.4.

The spu-priority option introduced in Junos OS Release 11.4R2.

Statement introduced on PTX Series Packet Transport Routers in Junos OS Release 12.1.

Change from 2 to 4 queues was made in Junos OS Release 12.3X48-D40 and in Junos OS Release 15.1X49-D70.

medium-high and medium-low priorities for spu-priority are deprecated and medium priority is added in Junos OS Release 19.1R1.