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Understanding CoS Scheduling on QFabric System Node Device Fabric (fte) Ports

 

Beginning with Junos OS Release 13.1, you can configure two-tier hierarchical scheduling (enhanced transmission selection, IEEE 802.1Qaz) on the fabric (fte) ports of QFabric system Node devices. Configuring CoS on Node device fabric interfaces provides increased control over traffic scheduling and helps to ensure predictable bandwidth consumption.

You can configure CoS on the following QFabric system interface types:

  • Node device access interfaces (xe interfaces)—Schedule traffic on the output queues of the 10-Gigabit Ethernet access ports using standard Node device CoS scheduling configuration components, as described elsewhere in the QFX Series documentation. You can configure different scheduling for different ports and output queues.

  • Node device fabric interfaces (fte interfaces)—Schedule traffic on the output queues of the 40-Gbps fabric interfaces that connect a Node device to a QFX3008-I or a QFX3600-I Interconnect device using standard Node device CoS scheduling configuration components. You can configure different scheduling for different interfaces and output queues.

This topic describes:

Hierarchical Scheduling Architecture on QFabric System Node Devices

CoS architecture on Node device access interfaces is the same as CoS architecture on standalone switch access interfaces. CoS architecture on Node device fabric interfaces is also the same as the CoS architecture on the access interfaces. You apply schedulers to queues (priorities), fc-sets (priority groups), and interfaces in the same hierarchical manner as described in Understanding CoS Hierarchical Port Scheduling (ETS).

You configure scheduling on Node device fabric interfaces (fte interfaces) using the same statements and configuration constructs that you use to configure scheduling on Node device access interfaces (xe interfaces). For example, on Node device fabric interfaces you can:

  • Define up to four fc-sets (three unicast, one multidestination)

    Note

    If the fabric interface handles strict-high priority traffic, you must define a separate fc-set (priority group) for strict-high priority traffic. Strict-high priority traffic cannot be mixed with traffic of other priorities in an fc-set. For example, you might choose to create different fc-sets for best-effort, lossless, strict-high priority, and multidestination traffic.

  • Map forwarding classes to fc-sets

  • Configure scheduling for each forwarding class (scheduler)

  • Configure scheduling for each fc-set (traffic control profile)

The differences in configuring CoS on Node device fabric interfaces compared to configuring CoS on Node device access interfaces are:

  • You specify a Node device fabric interface instead of a Node device access interface when you apply CoS to an interface.

  • You cannot attach classifiers, congestion notification profiles, or rewrite rules to fabric interfaces. Also, you cannot configure buffer settings on fabric interfaces. You can only attach fc-sets and traffic control profiles.

Default Scheduling on Node Device Fabric Interfaces

Default scheduling on Node device fabric interfaces is the same as default scheduling on Node device access interfaces. Only the default forwarding classes (best-effort, network-control, fcoe, no-loss, and multidestination) receive port bandwidth, based on the default minimum guaranteed bandwidth (transmit rate) scheduler settings for each default forwarding class.

To transport traffic on Node device fabric interfaces, the system organizes the default forwarding classes into three class groups. Class groups are not user-configurable. The three class groups are:

  • Unicast—All traffic in the default forwarding classes best-effort, network-control, fcoe, and no-loss belong to this default class group.

  • Multidestination—All traffic in the default forwarding class mcast belongs to this default class group.

  • Strict-high priority—There is no default strict-high priority forwarding class, so there is no default strict-high priority class group and there is no default configuration for strict-high priority traffic.

    Note

    If you configure strict-high priority forwarding classes, you must also configure an fc-set (priority group) for strict-high priority traffic, map the strict-high priority forwarding classes to the strict-high priority fc-set, create a scheduler for the strict-high priority traffic and map it to the strict-high priority forwarding classes, create a traffic control profile for the strict-high priority traffic, and apply the strict-high priority fc-set and traffic control profile to the appropriate fabric interfaces.

The default forwarding classes receive port bandwidth based on their default transmit rate settings (weights). Forwarding classes that are not default forwarding classes receive no default bandwidth.

Default class group scheduling uses weighted round-robin (WRR) scheduling, in which each class group receives a portion of the total available fabric interface bandwidth based on the class group traffic type, as shown in Table 1. Within each class group, the scheduler bandwidth allocation for individual forwarding classes is based on the default transmit rate for each forwarding class.

Table 1: Class Group Default Scheduling Properties and Membership on Node Device Fabric Interfaces

Class Group

Forwarding Class Mapping and Bandwidth Allocation (Default Transmit Rate)

Class Group Scheduling Properties (Weight)

Unicast

  • best-effort (5%)

  • fcoe (35%)

  • no-loss (35%)

  • network-control (5%)

Traffic in the unicast class group receives an 80% weight in the weighted round-robin (WRR) calculations. After the strict-high priority class group has been served, the unicast class group receives 80% of the remaining fabric bandwidth. (If more bandwidth is available, the unicast class group can use more bandwidth.)

Multidestination

  • mcast (20%)

Traffic in the multidestination class group receives a 20% weight in the WRR calculations. After the strict-high priority class group has been served, the multidestination class group receives 20% of the remaining fabric bandwidth. (If more bandwidth is available, the multidestination class group can use more bandwidth.)

Note

Strict-high priority traffic is served first, before any other traffic is served. Strict-high priority traffic receives all of the bandwidth it needs to empty its queues and therefore can starve other types of traffic during periods of high-volume strict-high priority traffic. Plan carefully and use caution when determining how much traffic to configure as strict-high priority traffic. We recommend that you always configure a shaping rate in the strict-high priority scheduler to set a maximum bandwidth limit for strict-high priority traffic.

Configuring Scheduling on Node Device Fabric Interfaces

If you do not want to use default CoS scheduling on Node device fabric interfaces, you can configure two-tier hierarchical scheduling (ETS) the same way that you configure ETS on Node device access interfaces.

Similarities Between Node Device Fabric Interface and Access Interface Scheduling

Configuring scheduling on a Node device fabric interface is similar to configuring scheduling on an access interface in many ways. In both cases, you configure:

  • Schedulers to specify the output scheduling for forwarding class traffic

  • Scheduler maps to map schedulers to forwarding classes

  • Forwarding classes (or use the default forwarding classes)

  • Forwarding class sets (groups of forwarding classes that require similar CoS treatment)

  • A separate fc-set for strict-high priority traffic (an fc-set cannot contain a mix of strict-high priority traffic and traffic with a different priority)

  • Traffic control profiles to specify the output scheduling for fc-sets

  • Traffic control profile and fc-set mapping to interfaces

On Node device fabric interfaces, you configure ETS in the same way, and ETS works the same way as on Node device access interfaces

In addition, strict-high priority queues are served first, and then the remaining port bandwidth is allocated to other traffic. Unless you configure a shaping rate in the scheduler for strict-high priority traffic, a strict-high priority queue can consume all of the port bandwidth and starve other queues, so we recommend that you always configure a shaping rate on strict-high priority traffic.

Differences Between Node Device Fabric Interface and Access Interface Scheduling

Configuring scheduling on a Node device fabric interface differs from configuring scheduling on an access interface in several ways. On fabric interfaces:

  • You cannot attach classifiers.

  • You cannot attach congestion notification profiles (flow control is applied automatically to lossless forwarding classes).

  • You cannot attach rewrite rules.

  • You cannot configure buffer settings.

  • You specify a Node device fabric interface name instead of a Node device access interface name when you apply CoS to an interface.