Priority Flow Control is a foundational mechanism used to provide lossless Ethernet behavior for RoCEv2. PFC operates on individual Ethernet priorities and allows a congested receiver or switch to pause transmission for the affected priority while permitting traffic in other classes to continue. Cisco's AI networking guidance explicitly states that PFC is enabled for RoCEv2 traffic to prevent packet drops caused by congestion.

This characteristic is critical for RDMA workloads because packet loss can trigger retransmission behavior and significantly degrade GPU collective communication performance. In an AI backend fabric, RoCEv2 is normally placed in a no-drop QoS class with PFC configured for that traffic class. ECN is commonly used alongside PFC to provide proactive congestion signaling, but PFC is the mechanism that directly prevents loss when buffers approach critical utilization.

Traffic shaping in option A may moderate burst rates but does not establish lossless transport. A high DSCP marking in option C provides classification and forwarding preference, not loss prevention. Weighted fair queuing in option D allocates scheduling bandwidth but also does not guarantee that packets will not be discarded during congestion.

Study Guide Reference: 3.1.a Congestion control mechanisms — PFC, ECN, ETS; 3.1.b RoCE/RoCEv2; 3.1.c QoS.

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