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Defining CoS Queue Schedulers for Port Scheduling

Schedulers define the CoS properties of output queues. You configure CoS properties in a scheduler, then map the scheduler to a forwarding class. Forwarding classes are in turn mapped to output queues. Classifiers map incoming traffic into forwarding classes based on IEEE 802.1p, DSCP, or EXP code points. CoS scheduling properties include the amount of interface bandwidth assigned to the queue, the priority of the queue, whether explicit congestion notification (ECN) is enabled on the queue, and the WRED packet drop profiles associated with the queue.

The parameters you configure in a scheduler define the following characteristics for the queues mapped to the scheduler:

  • priority—One of three bandwidth priorities that queues associated with a scheduler can receive:

    • low—The scheduler has low priority.

    • high—The scheduler has high priority. High priority traffic takes precedence over low priority traffic.

    • strict-high—The scheduler has strict-high priority. Strict-high priority queues receive preferential treatment over low-priority queues and receive all of their configured bandwidth before low-priority queues are serviced. Low-priority queues do not transmit traffic until strict-high priority queues are empty.


      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.

  • transmit-rate—Minimum guaranteed bandwidth, also known as the committed information rate (CIR), set as a percentage rate or as an absolute value in bits per second. By default, the transmit rate also determines the amount of excess (extra) port bandwidth the queue can share if you do not explicitly configure an excess rate. Extra bandwidth is allocated among the queues on the port in proportion to the transmit rate of each queue. Except on QFX10000 switches, you can configure shaping-rate to throttle the rate of packet transmission. On QFX10000 switches, on queues that are not strict-high priority queues, you can configure a transmit rate as exact, which shapes the transmission by setting the transmit rate as the maximum bandwidth the queue can consume on the port.


    On QFX10000 switches, oversubscribing all 8 queues configured with the transmit rate exact (shaping) statement at the [edit class-of-service schedulers scheduler-name] hierarchy level might result in less than 100 percent utilization of port bandwidth.

    On strict-high priority queues, the transmit rate sets the amount of bandwidth used for strict-high priority forwarding; traffic in excess of the transmit rate is treated as best-effort traffic that receives the queue excess rate.


    Include the preamble bytes and interframe gap (IFG) bytes as well as the data bytes in your bandwidth calculations.

  • excess-rate—Percentage of extra bandwidth (bandwidth that is not used by other queues) a low-priority queue can receive. If not set, the switch uses the transmit rate to determine extra bandwidth sharing. You cannot set an excess rate on a strict-high priority queue.

  • drop-profile-map—Drop profile mapping to a packet loss priority to apply WRED to the scheduler and control packet drop for different packet loss priorities during periods of congestion.

  • buffer-size—Size of the queue buffer as a percentage of the dedicated buffer space on the port, or as a proportional share of the dedicated buffer space on the port that remains after the explicitly configured queues are served.

  • explicit-congestion-notification—ECN enable on a best-effort queue. ECN enables end-to-end congestion notification between two ECN-enabled endpoints on TCP/IP based networks. ECN must be enabled on both endpoints and on all of the intermediate devices between the endpoints for ECN to work properly. ECN is disabled by default.


Do not configure drop profiles for the fcoe and no-loss forwarding classes. FCoE and other lossless traffic queues require lossless behavior. Use priority-based flow control (PFC) to prevent frame drop on lossless priorities.

To apply scheduling properties to traffic, map schedulers to forwarding classes using a scheduler map, and then apply the scheduler map to interfaces. Using different scheduler maps, you can map different schedulers to the same forwarding class on different interfaces, to apply different scheduling to that traffic on different interfaces.

To configure a scheduler using the CLI:

  1. Name the scheduler and set the minimum guaranteed bandwidth for the queue; optionally, set a maximum bandwidth limit (shaping rate) on a low priority queue by configuring either shaping-rate (except on QFX10000 switches) or the exact option (only on QFX10000 switches):

  2. Set the amount of excess bandwidth a low-priority queue can share:

  3. Set the queue priority:

  4. Specify drop profiles for packet loss priorities using a drop profile map:

  5. Configure the size of the buffer space for the queue:

  6. Enable ECN, if desired (on best-effort traffic only):

  7. Configure a scheduler map to map the scheduler to a forwarding class, which applies the scheduler’s properties to the traffic in that forwarding class:

  8. Assign the scheduler map and its associated schedulers to one or more interfaces.