Any packet entering the fabric with any DSCP/DOT1P marking can be mapped to any queue of the interface and enabling PFC on this queue makes it lossless. With PFC in place, when congestion is detected in the queue, a pause frame is sent back to the sender, signaling it to temporarily halt sending traffic of that priority. This mechanism effectively prevents packet drops, ensuring lossless transmission for traffic of particular priority.

Beyond PFC, there’s another layer of congestion management—Explicit Congestion Notification (ECN). With ECN, we can define buffer thresholds, exceeding which Congestion Notification (ECN-CNP) packets are sent to the sender, prompting it to reduce the transmission rate and proactively avoid congestion.

At this stage, we’ve ensured that our priority traffic is lossless. Moving into the egress phase, we can further enhance performance by prioritizing this traffic over others, even under congestion. SONiC provides scheduling algorithms like Deficit Weighted Round Robin (DWRR), Weighted Round Robin (WRR), and Strict Priority Scheduling (STRICT). By binding priority queues to these schedulers, the system can ensure that higher-priority traffic is transmitted preferentially, either in a weighted manner (for WRR/DWRR) or with absolute priority (for STRICT).

In summary, through PFC, ECN, and advanced scheduling techniques, SONiC ensures that high-priority traffic from GPU servers is not only lossless but also prioritized during both congestion and egress phases.