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Understanding CoS Forwarding Class Sets (Priority Groups)

You can configure up to three unicast forwarding class sets and one multicast forwarding class set. Only unicast forwarding classes can belong to unicast forwarding class sets. Only multicast forwarding classes can belong to the multicast forwarding class set.

• You must create a separate forwarding class set for the strict-high priority queue.
• Only one forwarding class set can contain strict-high priority queues.
• Strict-high priority queues cannot belong to the same forwarding class set as queues that are not strict-high priority.
• A strict-high priority queue cannot belong to a multidestination forwarding class set.
• You cannot configure a guaranteed minimum bandwidth (guaranteed rate) for a forwarding class set that includes a strict-high priority queue. (You also cannot configure a guaranteed minimum bandwidth for a strict-high queue.)
• 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.

You must use hierarchical scheduling to define CoS for output queues. The two-tier hierarchical scheduler defines bandwidth resources for the priority group, and then allocates those resources among the priorities that belong to the priority group.

If you do not explicitly configure forwarding class sets, the system automatically creates a default forwarding class set that contains all of the forwarding classes on the switch. The system assigns 100 percent of the port output bandwidth to the default forwarding class set. Ingress traffic is classified based on the default classifier settings. The forwarding classes (queues) in the default forwarding class set receive bandwidth based on the default scheduler settings. Forwarding classes that are not part of the default scheduler receive no bandwidth. The default priority group is transparent. It does not appear in the configuration and is used for Data Center Bridging Capability Exchange Protocol (DCBX) advertisement.

When you explicitly configure forwarding class sets and map them to an interface, any forwarding class that you do not map to a forwarding class set receives no guaranteed bandwidth on that interface. Forwarding classes that belong to the default forwarding class set might receive bandwidth if the other forwarding class sets are not using all of the port bandwidth. However, the amount of bandwidth forwarding classes that are not in explicitly configured forwarding class sets receive is not guaranteed. The bandwidth for the default forwarding class depends on whether extra port bandwidth is available and therefore is not deterministic.

To guarantee bandwidth for forwarding classes in a predictable manner, be sure to map all forwarding classes that you expect to carry traffic on an interface to a forwarding class set and map the forwarding class set to the interface.