Understanding CoS Forwarding Class Sets (Priority Groups)
A forwarding class set is the Junos OS configuration construct that equates to a priority group in enhanced transmission selection (ETS, described in IEEE 802.1Qaz). The switch implements ETS using a two-tier hierarchical scheduler.
A priority group is a group of forwarding classes. Each forwarding class is mapped to an output queue and an IEEE 802.1p priority (code points). Classifying traffic into a forwarding class based on its code points, and mapping the forwarding class to a queue, defines the traffic assigned to that queue. The forwarding classes that belong to a priority group share the port bandwidth allocated to that priority group. The traffic mapped to forwarding classes in one priority group usually shares similar traffic-handling requirements.
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.
If you configure a strict-high priority forwarding class (you can configure only one strict-high priority forwarding class), you must observe the following rules when configuring forwarding class sets:
You must create a separate forwarding class set for the strict-high priority forwarding class.
Only one forwarding class set can contain the strict-high priority forwarding class.
A strict-high priority forwarding class cannot belong to the same forwarding class set as forwarding classes that are not strict-high priority.
A strict-high priority forwarding class 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 forwarding class. (You also cannot configure a guaranteed minimum bandwidth for a strict-high forwarding class.)
We recommend that you always apply a shaping rate to a strict-high priority forwarding class to prevent it from starving the queues mapped to other forwarding classes. If you do not apply a shaping rate to limit the amount of bandwidth a strict-high priority forwarding class can use, then the strict-high priority forwarding class can use all of the available port bandwidth and starve other forwarding classes on the port.
You must use hierarchical scheduling if you explicitly configure CoS. The two-tier hierarchical scheduler defines bandwidth resources for the forwarding class set (priority group), and then allocates those resources among the forwarding classes (priorities) that belong to the forwarding class set.
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 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 (except on OCX Series switches, which do not support DCBX).
When you explicitly configure forwarding class sets and apply them to interfaces, on those interfaces, forwarding classes that you do not map to a forwarding class set receive no guaranteed bandwidth. 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 received by forwarding classes that are not members of a forwarding class set is not guaranteed. In this case, the bandwidth a forwarding class receives if it is not a member of a forwarding class set depends on whether unused 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 apply the forwarding class set to the interface.