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Understanding Default CoS Scheduling and Classification

If you do not configure hierarchical scheduling on an interface, the switch uses the default classifiers for ingress traffic and the default schedulers for egress traffic. Default scheduling and classification handle all traffic types (best-effort, FCoE, no-loss, network-control, and multidestination traffic).

Hierarchical scheduling groups egress queues (priorities, configured as forwarding classes) into priority groups (forwarding class sets). If you use only the default traffic scheduling and classification, the switch automatically creates a default priority group that contains all of the priorities (forwarding classes, which represent output queues) and assigns 100 percent of the port output bandwidth to that priority group. The forwarding classes (queues) in the default forwarding class set receive bandwidth based on the default classifier settings. The default priority group is transparent. It does not appear in the configuration and is used for Data Center Bridging Capability Exchange (DCBX) protocol advertisement.

Note: If you explicitly configure one or more priority groups on an interface, any forwarding class that is not assigned to a priority group on that interface receives no bandwidth. This means that if you configure hierarchical scheduling on an interface, every forwarding class (priority) that you want to forward traffic on that interface must belong to a forwarding class set (priority group).

Default Classification

The default classifiers assign unicast and multicast best-effort and network-control ingress traffic to forwarding classes and loss priorities. The QFX Series applies default unicast IEEE 802.1, unicast DSCP, and multidestination classifiers to each interface that does not have explicitly configured classifiers. If you explicitly configure one type of classifier but not other types of classifiers, the system uses only the configured classifier and does not use default classifiers for other types of traffic. There are two different default unicast IEEE 802.1 classifiers, a trusted classifier for ports that are in trunk mode or tagged-access mode, and an untrusted classifier for ports that are in access mode.

Table 1 shows the default mapping of IEEE 802.1 code-point values to unicast forwarding classes and loss priorities for ports in trunk mode or tagged-access mode.

Table 2 shows the default mapping of IEEE 802.1p code-point values to unicast forwarding classes and loss priorities for ports in access mode (all incoming traffic is mapped to best-effort forwarding classes).

Table 3 shows the default mapping of IEEE 802.1 code-point values to multidestination (multicast, broadcast, and destination lookup fail traffic) forwarding classes and loss priorities.

Table 4 shows the default mapping of DSCP code-point values to unicast forwarding classes and loss priorities for DSCP IP and DCSP IPv6.

Note: There are no default DSCP IP or IPv6 multidestination classifiers for multidestination traffic. DSCP IPv6 multidestination classifiers are not supported for multidestination traffic.

Default Scheduling

The default schedulers allocate egress bandwidth resources to unicast and multicast best-effort and network-control egress traffic as shown in Table 5:

Note: The minimum guaranteed bandwidth rate also determines the amount of excess (extra) bandwidth that the queue can share. Extra bandwidth is allocated to queues in proportion to the minimum guaranteed bandwidth rate of each queue.

Default DCBX Advertisement

When you configure hierarchical scheduling on an interface, DCBX advertises each priority group, the priorities in each priority group, and the bandwidth properties of each priority and priority group.

If you do not configure hierarchical scheduling on an interface, DCBX advertises the automatically created default priority group and its priorities. DCBX also advertises the default bandwidth allocation of the priority group, which is 100 percent of the port bandwidth.

Default Scheduling and Classification Summary

• Default classifiers classify ingress traffic.
• Default schedulers schedule egress traffic.
• DCBX advertises a single default priority group with 100 percent of the port bandwidth allocated to that priority group. All priorities (forwarding classes) are assigned to the default priority group and receive bandwidth based on their default schedulers. The default priority group is generated automatically and is not user-configurable.