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Understanding CoS Forwarding Classes

Default Forwarding Classes

Table 1 shows the four default forwarding classes defined for unicast traffic, and Table 2 shows the four default forwarding classes defined for multicast traffic.

If desired, you can rename the forwarding classes associated with the queues supported on your switch. Assigning a new class name to an output queue does not alter the default classification or scheduling that is applicable to that queue. CoS configurations can be quite complicated, so unless it is required by your scenario, we recommend that you not alter the default class names or queue number associations.

Note: Mirrored traffic is always sent to the queue that corresponds to the multidestination forwarding class. The switched copy of the mirrored traffic is forwarded with the priority determined by the behavior aggregate classification process.

Forwarding Class Configuration Rules

Take the following rules into account when you configure forwarding classes:

• Queue Assignment Rules
• Scheduling Rules
• Rewrite Rules

Queue Assignment Rules

The following rules govern queue assignment:

• CoS configurations that specify more queues than the switch can support are not accepted. The commit operation fails with a detailed message that states the total number of queues available.
• All default CoS configurations are based on queue number. The name of the forwarding class that appears in the default configuration is the forwarding class currently associated with that queue.
• Only unicast forwarding classes can be mapped to unicast queues (0 through 7), and only multidestination forwarding classes can be mapped to multidestination queues (8 through 11).
• Strict-high priority queues cannot be mapped to multidestination forwarding classes. (Strict-high priority traffic cannot be mapped to queues 8 through 11).
• If you map more than one forwarding class to a queue, all of the forwarding classes mapped to the same queue must have the same packet drop attribute (all of the forwarding classes must be lossy, or all of the forwarding classes mapped to a queue must be lossless).

In addition, if you configure a strict-high priority queue, we recommend that you always apply a shaping rate to prevent the strict-high priority queue 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.

Scheduling Rules

When you define a forwarding class that is used on the switch (the behavior aggregate classifier has a forwarding class and you expect traffic for the forwarding class), you must also define a scheduling policy for the forwarding class. Defining a scheduling policy means:

• Mapping a scheduler to the forwarding class in a scheduler map
• Including the forwarding class in a forwarding class set
• Associating the scheduler map with a traffic control profile
• Attaching the traffic control profile to a forwarding class set and an interface

Rewrite Rules

On each physical interface, either all forwarding classes that are being used on the interface must have rewrite rules configured, or no forwarding classes that are being used on the interface can have rewrite rules configured. On any physical port, do not mix forwarding classes with rewrite rules and forwarding classes without rewrite rules.

Lossless Transport Support

The switch supports up to six lossless forwarding classes. For lossless transport, you must enable PFC on the IEEE 802.1p code point of lossless forwarding classes. The following limitations apply to support lossless transport:

• The external cable length from the switch or QFabric system Node device to other devices cannot exceed 300 meters.
• The internal cable length from the QFabric system Node device to the QFabric system Interconnect device cannot exceed 150 meters.
• For FCoE traffic, the interface maximum transmission unit (MTU) must be at least 2180 bytes to accommodate the packet payload, headers, and checks.
• Changing any portion of a PFC configuration on a port blocks the entire port until the change is completed. After a PFC change is completed, the port is unblocked and traffic resumes. Changing the PFC configuration means any change to a congestion notification profile that is configured on a port (enabling or disabling PFC on a code point, changing the MRU or cable-length value, or specifying an output flow control queue). Blocking the port stops ingress and egress traffic, and causes packet loss on all queues on the port until the port is unblocked.

Note: Junos OS Release 12.2 introduces changes to the way lossless forwarding classes (the fcoe and no-loss forwarding classes) are handled. In Junos OS Release 12.1, both explicitly configuring the fcoe and no-loss forwarding classes, and using the default configuration for these forwarding classes, resulted in the same lossless behavior for traffic mapped to those forwarding classes. However, in Junos OS Release 12.2, if you explicitly configure the fcoe or the no-loss forwarding class, that forwarding class is no longer treated as a lossless forwarding class. Traffic mapped to these forwarding classes is treated as lossy (best-effort) traffic. This is true even if the explicit configuration is exactly the same as the default configuration. If your CoS configuration from Junos OS Release 12.1 or earlier includes the explicit configuration of the fcoe or the no-loss forwarding class, then when you upgrade to Junos OS Release 12.2, those forwarding classes are not lossless. To preserve the lossless treatment of these forwarding classes, delete the explicit fcoe and no-loss forwarding class configuration before you upgrade to Junos OS Release 12.2. See Overview of CoS Changes Introduced in Junos OS Release 12.2 for detailed information about this change and how to delete an existing lossless configuration. In Junos OS Release 12.3, the default behavior of the fcoe and no-loss forwarding classes is the same as in Junos OS Release 12.2. However, in Junos OS Release 12.3, you can configure up to six lossless forwarding classes. All explicitly configured lossless forwarding classes must include the new no-loss packet drop attribute or the forwarding class is lossy.