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Understanding CoS Output Queue Schedulers

 

Output queue scheduling defines the class-of-service (CoS) properties of output queues. Output queues are mapped to forwarding classes, and classifiers map incoming traffic into forwarding classes based on IEEE 802.1p or DSCP code points. Output queue properties include the amount of interface bandwidth assigned to the queue, the size of the memory buffer allocated for storing packets, the priority of the queue, and the weighted random early detection (WRED) drop profiles associated with the queue. Queue scheduling works with priority group scheduling to create a two-tier hierarchical scheduler.

The hierarchical scheduler allocates port bandwidth to a group of queues (forwarding classes) called a priority group (forwarding class set), and queue scheduling determines the portion of the priority group’s bandwidth that a particular queue can use. So the first scheduling tier is allocating port bandwidth to a forwarding class set, and the second scheduling tier is allocating forwarding class set bandwidth to forwarding classes (queues).

Scheduler maps associate queue schedulers with forwarding classes. The queue mapped to a forwarding class receives the scheduling resources assigned to that forwarding class. You associate a scheduler map with a traffic control profile, and then associate the traffic control profile with a forwarding class set (priority group) and a port interface to apply scheduling to a port. In conjunction with the priority group scheduling configured in the traffic control profile, queue scheduling configures the packet schedulers and weighted random early detection (WRED) packet drop processes for queues.

Note

When you configure bandwidth for a queue or a priority group, the switch considers only the data as the configured bandwidth. The switch does not account for the bandwidth consumed by the preamble and the interframe gap (IFG). Therefore, when you calculate and configure the bandwidth requirements for a queue or for a priority group, consider the preamble and the IFG as well as the data in the calculations.

Output Queue Scheduling Components

Table 1 provides a quick reference to the scheduler components you can configure to determine the bandwidth properties of output queues (forwarding classes), and Table 2 provides a quick reference to some related scheduling configuration components.

Table 1: Output Queue Scheduler Components

Output Queue Scheduler Component

Description

Buffer size

Sets the size of the queue buffer.

See Understanding CoS Buffer Configuration.

Drop profile map

Maps a drop profile to a loss priority. Drop profile map components include:

  • Drop profile—Sets the probability of dropping packets as the queue fills up.

  • Loss priority—Sets the traffic loss priority to which a drop profile applies.

See Configuring CoS Drop Profile Maps.

Explicit congestion notification

Enables explicit congestion notification (ECN) on the queue.

See Understanding CoS Explicit Congestion Notification.

Priority

Sets the scheduling priority applied to the queue.

See Defining CoS Queue Scheduling Priority.

Shaping rate

Sets the maximum bandwidth the queue can consume.

Tip: On QFX5200 Series switches, a granularity of 64kbps is supported for the shaping rate.

See Understanding CoS Priority Group Shaping and Queue Shaping (Maximum Bandwidth).

Transmit rate

Sets the minimum guaranteed bandwidth for the queue. Extra bandwidth is shared among queues in proportion to the minimum guaranteed bandwidth of each queue. See Understanding CoS Priority Group and Queue Guaranteed Minimum Bandwidth.



Table 2: Other Scheduling Components

Other Scheduling Components

Description

Forwarding class

Maps traffic to an output queue. Classifiers map forwarding classes to IEEE 802.1p, DSCP, or EXP code points. A forwarding class, an output queue, and code point bits are mapped to each other and identify the same traffic. (The code point bits identify incoming traffic. Classifiers assign traffic to forwarding classes based on the code point bits. Forwarding classes are mapped to output queues. This mapping determines the output queue each class of traffic uses on the switch egress interfaces.)

Output queue

Buffers traffic before the switch forwards the traffic out the egress interface. Output queues are mapped to forwarding classes. The switch applies CoS properties defined in schedulers to output queues, by mapping forwarding classes to schedulers in scheduler maps. The queue mapped to the forwarding class has the CoS properties defined in the scheduler mapped to that forwarding class.

Scheduler map

Maps schedulers to forwarding classes (forwarding classes are mapped to queues, so a forwarding class represents a queue, and the scheduler mapped to a forwarding class determines the CoS properties of the output queue mapped to that forwarding class).

Traffic control profile

Configures scheduling for the forwarding class set (priority group), and associates a scheduler map with the forwarding class set to apply queue scheduling to the forwarding classes in the forwarding class set. Extra port bandwidth is shared among forwarding class sets in proportion to the minimum guaranteed bandwidth of each forwarding class set.

Forwarding class set

Name of a priority group. You map forwarding classes to forwarding class sets. A forwarding class set consists of one or more forwarding classes.

Default Schedulers

Each forwarding class requires a scheduler to set the CoS properties of the forwarding class and its output queue. You can use the default schedulers or you can define new schedulers for the associated forwarding classes. For any other forwarding class, you must explicitly configure a scheduler. For more information, see Default Scheduling.

Transmit Rate (Minimum Guaranteed Bandwidth)

The transmit rate determines the minimum guaranteed bandwidth for each forwarding class. The switch applies the minimum bandwidth guarantee to the output queue mapped to the forwarding class. The transmit rate also determines how much excess (extra) bandwidth each low-priority queue can share; each queue shares extra bandwidth in proportion to its transmit rate. You specify the rate in bits per second as a fixed value such as 1 Mbps or as a percentage of the total forwarding class set minimum guaranteed bandwidth (the guaranteed rate set in the traffic control profile). Either the default scheduler or a scheduler you configure allocates a portion of the outgoing interface bandwidth to each forwarding class in proportion to the transmit rate.

Note

For transmit rates below 1 Gbps, we recommend that you configure the transmit rate as a percentage instead of as a fixed rate. This is because the system converts fixed rates into percentages and may round small fixed rates to a lower percentage. For example, a fixed rate of 350 Mbps is rounded down to 3 percent.

You cannot configure a transmit rate for a strict-high priority queue. Queues with a configured transmit rate cannot be included in a forwarding class set that has a strict-high priority queue (you cannot mix strict-high priority queues and queues that are not strict-high priority in the same forwarding class set).

The allocated bandwidth can exceed the configured minimum rate if additional bandwidth is available from other queues in the forwarding class set that are not using all of their allocated bandwidth. During periods of congestion, the configured transmit rate is the guaranteed bandwidth minimum for the queue. This behavior enables you to ensure that each queue receives the amount of bandwidth appropriate to its level of service and is also able to share unused bandwidth.

Note

Configuring the minimum guaranteed bandwidth (transmit rate) for a forwarding class does not work unless you also configure the minimum guaranteed bandwidth (guaranteed rate) for the forwarding class set in the traffic control profile.

Additionally, the sum of the transmit rates of the queues in a forwarding class set should not exceed the guaranteed rate for the forwarding class set. (You cannot guarantee a combined minimum bandwidth for the queues that is greater than the minimum bandwidth guaranteed for the entire set of queues.)

For more information, see Understanding CoS Priority Group and Queue Guaranteed Minimum Bandwidth.

Sharing Extra Bandwidth

Extra bandwidth is available to low-priority queues when a forwarding class set does not use its full amount of minimum guaranteed bandwidth (guaranteed-rate). Extra bandwidth is shared among the forwarding classes in a forwarding class set in proportion to the minimum guaranteed bandwidth (transmit-rate) of each queue.

For example, in a forwarding class set, Queue A has a transmit rate of 1 Gbps, Queue B has a transmit rate of 1 Gbps, and Queue C has a transmit rate of 2 Gbps. After servicing the minimum guaranteed bandwidth of these queues, the forwarding class set has an extra 2 Gbps of bandwidth available, and all three queues still have packets to forward. The queues receive the extra bandwidth in proportion to their transmit rates, so Queue A receives an extra 500 Mbps, Queue B receives an extra 500 Mbps, and Queue C receives an extra 1 Gbps.

Shaping Rate (Maximum Bandwidth)

The shaping rate sets the maximum bandwidth that a forwarding class can consume. You specify the rate in bits per second as a fixed value, such as 3 Mbps or as a percentage of the total forwarding class set maximum bandwidth (the shaping rate set in the traffic control profile).

The maximum bandwidth for a queue depends on the total bandwidth available to the forwarding class set to which the queue belongs, and on how much bandwidth the other queues in the forwarding class set consume.

Note

On QFabric systems, if any queue that contains outgoing packets does not transmit packets for 12 consecutive seconds, the port automatically resets. A strict-high priority queue (or several queues with higher priorities than the starved queue) can consume all of the port bandwidth and prevent another queue from transmitting packets. To prevent a queue from being starved for bandwidth, you can configure a shaping rate on the queue or queues to prevent them from consuming all of the port bandwidth.

Note

We recommend that you always configure a shaping rate in the scheduler for strict-high priority queues to prevent them from starving other queues.

For more information, see Understanding CoS Priority Group Shaping and Queue Shaping (Maximum Bandwidth).

Scheduling Priority

Scheduling priority determines the order in which an interface transmits traffic from its output queues. This ensures that queues containing important traffic receive prioritized access to the outgoing interface bandwidth. The priority setting in the scheduler determines the priority for the queue.

For more information, see Defining CoS Queue Scheduling Priority.

Scheduler Drop-Profile Maps

Drop-profile maps associate drop profiles with queue schedulers and packet loss priorities (PLPs). Drop profiles set thresholds for dropping packets during periods of congestion, based on the queue fill level and a percentage probability of dropping packets at the specified queue fill level. At different fill levels, a drop profile sets different probabilities of dropping a packet during periods of congestion.

Classifiers assign incoming traffic to forwarding classes (which are mapped to output queues), and also assign a PLP to the incoming traffic. The PLP can be low, medium-high, or high. You can classify traffic with different PLPs into the same forwarding class to differentiate treatment of traffic within the forwarding class.

In a drop profile map, you can configure a different drop profile for each PLP and associate (map) the drop profiles to a queue scheduler. A scheduler map maps the queue scheduler to a forwarding class (output queue). Traffic classified into the forwarding class uses the drop characteristics defined in the drop profiles that the drop profile map associates with the queue scheduler. The drop profile the traffic uses depends on the PLP that the classifier assigns to the traffic. (You can map different drop profiles to the forwarding class for different PLPs.)

In summary:

  • Classifiers assign one of three PLPs (low, medium-high, high) to incoming traffic when classifiers assign traffic to a forwarding class.

  • Drop profiles set thresholds for packet drop at different queue fill levels.

  • Drop profile maps associate a drop profile with each PLP, and map the drop profiles to schedulers.

  • Scheduler maps map schedulers to forwarding classes, and forwarding classes are mapped to output queues. The scheduler mapped to a forwarding class determines the CoS characteristics of the output queue mapped to the forwarding class, including the drop profile mapping.

Buffer Size

Most of the total system buffer space is divided into two buffer pools, shared buffers and dedicated buffers. Shared buffers are a global pool that the ports share dynamically as needed. Dedicated buffers are a reserved portion of the buffer pool that is distributed evenly to all of the ports. Each port receives an equal allocation of dedicated buffer space. The dedicated buffer allocation to ports is not configurable because it is reserved for the ports.

The queue buffers are allocated from the dedicated buffer pool assigned to the port. By default, ports divide their allocation of dedicated buffers among the egress queues in the same proportion as the default scheduler sets the minimum guaranteed transmission rates (transmit-rate) for traffic. Only the queues included in the default scheduler receive dedicated buffers.

If you do not use the default configuration, you can explicitly configure the queue buffer size in either of two ways:

  • As a percentage—The queue receives the specified percentage of dedicated port buffers when the queue is mapped to the scheduler and the scheduler is mapped to a port.

  • As a remainder—After the port services the queues that have an explicit percentage buffer size configuration, the remaining port dedicated buffer space is divided equally among the other queues to which a scheduler is attached. (No default or explicit scheduler means no dedicated buffer allocation for the queue.) If you configure a scheduler and you do not specify a buffer size as a percentage, remainder is the default setting.

Note

The total of all of the explicitly configured buffer size percentages for all of the queues on a port cannot exceed 100 percent.

For a complete discussion about queue buffer configuration in the context of ingress and egress port buffer configuration, see Understanding CoS Buffer Configuration.

Explicit Congestion Notification

Explicit congestion notification (ECN) notifies networks about congestion with the goal of reducing packet loss and delay by making the sending device decrease the transmission rate until the congestion clears, without dropping packets. ECN enables end-to-end congestion notification between two endpoints on TCP/IP based networks. ECN is disabled by default.

For more information, see Understanding CoS Explicit Congestion Notification.

Scheduler Maps

A scheduler map associates a forwarding class with a scheduler configuration. After configuring a scheduler, you must include it in a scheduler map, associate the scheduler map with a traffic control profile, and then associate the traffic control profile with an interface and a forwarding class set to implement the configured queue scheduling.

You can associate up to four user-defined scheduler maps with traffic control profiles. For more information, see Default Schedulers Overview.