How to Configure Priority Scheduling on Junos Routers

Each scheduler is assigned a priority. By extension, because each queue is linked to a scheduler, each queue has a priority associated with it. When packets are sent to an outbound interface for transmission, they’re stored in queues, as defined by their forwarding class.

Junos services these queues based on their priority. If you want to ensure that a particular queue is serviced ahead of others, configure a higher priority for that queue’s scheduler.

For example, the video traffic in your network may be high priority. The business-critical traffic in the network may be important but somewhat lower in priority than the video traffic, and the noncritical traffic may be lower still. Finally, the default traffic may be of the lowest priority. To reflect this prioritization in the configuration:

[edit class-of-service]
schedulers {
  cos-sched-voice {
   priority strict-high;
  }
  cos-sched-video {
   transmit-rate percent 40;
   buffer-size percent 40;
   priority high;
  }
  cos-sched-buscrit {
   transmit-rate percent 30;
   buffer-size percent 10;
   priority medium-high;
  }
  cos-sched-noncrit {
   transmit-rate percent 10;
   buffer-size percent 10;
   priority medium-low;
  }
  cos-sched-default {
   transmit-rate remainder;
   buffer-size remainder;
   priority low;
  }
}

When you configured the scheduler to handle voice traffic, you configured the priority as strict-high, despite not having yet introduced the notion of priority scheduling — and that was really just an instance of this same concept.

Once you set these priorities, Junos uses them to determine the order in which queues are serviced. On a particular physical interface, the output queues are serviced as follows:

1. All high-priority queues currently in profile are serviced.

   
   

   In profile means that the traffic fits within the allocated bandwidth for that particular queue. Multiple high-priority queues are serviced in weighted round-robin fashion. High-priority queues include strict-high priority queues, which are considered to always be in profile.

   
   



2. All medium-high priority queues currently in profile are serviced.

   
   

   Again, multiple queues of the same priority are serviced in weighted round-robin fashion.

   
   



3. All medium-low priority queues in profile are serviced.

   
   



4. All low-priority queues in profile are serviced.

   
   



5. All high-priority queues that are currently out of profile and are not rate-limited are serviced.

   
   

   Multiple queues are serviced in weighted round-robin fashion.

   
   



6. All medium-high priority queues currently out of profile are serviced.

   
   



7. All medium-low priority queues currently out of profile are serviced.

   
   



8. All low-priority queues currently out of profile are serviced.

   
   

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Source:http://www.dummies.com/how-to/content/how-to-configure-priority-scheduling-on-junos-rout.html