Defining CoS Queue Scheduling Priority


You can configure the scheduling priority of individual queues by specifying the priority in a scheduler, and then associating the scheduler with a queue by using a scheduler map. On QFX5100, QFX5200, EX4600, QFX3500, and QFX3600 switches, and on QFabric systems, queues can have one of two bandwidth scheduling priorities, strict-high priority or low priority. On QFX10000 Series switches, queues can also be configured as high priority.


By default, all queues are low priority queues.

The switch services low priority queues after servicing any queue that has strict-high priority traffic or high priority traffic. Strict-high priority queues receive preferential treatment over all other queues and receive all of their configured bandwidth before other queues are serviced. Low-priority queues do not transmit traffic until strict-high priority queues are empty, and receive the bandwidth that remains after the strict-high queues have been serviced. High priority queues receive preference over low priority queues.

Different switches handle traffic configured as strict-high priority traffic in different ways:

  • QFX5100, QFX5200, QFX3500, QFX3600, and EX4600 switches, and QFabric systems—You can configure only one queue as a strict-high priority queue.

    On these switches, we recommend that you always apply a shaping rate to strict-high priority queues to prevent them from starving other queues. If you do not apply a shaping rate to limit the amount of bandwidth a strict-high priority queue can use, then the strict-high priority queue can use all of the available port bandwidth and starve other queues on the port.

  • QFX10000 switches—You can configure as many queues as you want as strict-high priority. However, keep in mind that too much strict-high priority traffic can starve low priority queues on the port.


    We strongly recommend that you configure a transmit rate on all strict-high priority queues to limit the amount of traffic the switch treats as strict-high priority traffic and prevent strict-high priority queues from starving other queues on the port. This is especially important if you configure more than one strict-high priority queue on a port. If you do not configure a transmit rate to limit the amount of bandwidth strict-high priority queues can use, then the strict-high priority queues can use all of the available port bandwidth and starve other queues on the port.

    The switch treats traffic in excess of the transmit rate as best-effort traffic that receives bandwidth from the leftover (excess) port bandwidth pool. On strict-high priority queues, all traffic that exceeds the transmit rate shares in the port excess bandwidth pool based on the strict-high priority excess bandwidth sharing weight of “1”, which is not configurable. The actual amount of extra bandwidth that traffic exceeding the transmit rate receives depends on how many other queues consume excess bandwidth and the excess rates of those queues.