QoS strict priority egress traffic scheduling

Egress traffic scheduling allows you to selectively manage traffic based on the forwarding queue to which it is mapped.

Strict priority scheduling (SP) scheduling is used to facilitate support for latency sensitive traffic. A strict priority scheduler drains all frames queued in the highest-priority queue before continuing on to service lower-priority traffic classes.

The following figure displays the frame scheduling order for an SP scheduler servicing two SP queues. The higher-numbered queue, SP2, has a higher priority.

The disadvantage of strict priority-based scheduling is that lower-priority traffic can be starved of any access.

The devices classify packets into one of eight internal priorities. For each egress port, there are 8 Virtual output queues (VOQ) allocated on each ingress TM core to support 8 priorities. SP queue input values map to traffic classes and range from 0 through 7. These are:

0 - No strict priority queue.
1 - Traffic Class 7 strict priority queue.
2 - Traffic Class 6 through 7 strict priority queues.
3 - Traffic Class 5 through 7 strict priority queues.
4 - Traffic Class 4 through 7 strict priority queues.
5 - Traffic Class 3 through 7 strict priority queues.
6 - Traffic Class 2 through 7 strict priority queues.
7 - Traffic Class 1 through 7 strict priority queues.

When configuring egress traffic scheduling you use credit request and grant mechanisms to perform QoS. The credit size is 1024B.