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Understanding the SPC High-Priority Queue

The Services Processing Card (SPC) on SRX1400, SRX3000 line, and SRX5000 line devices provides processing power to run integrated services such as firewall, IPsec, and IDP. 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 different priority queues on the SPC. The Services Processing Unit (SPU) draws packets from the higher priority queues and only draws packets from lower priority queues when the higher priority queues are empty. This feature can reduce overall latency for real-time traffic, such as voice traffic.

To designate packets for the high-priority, medium-priority, or low-priority queues, use the spu-priority configuration statement at the [edit class-of-service forwarding-classes class] hierarchy level. A value of high places packets into the high-priority queue, a value of medium places packets into the medium-priority queue, and a value of low places packets into the low-priority queue.