What Is Priority Flow Control?_
PFC, defined by the IEEE 802.1Qbb standard, operates on a per-priority basis, allowing a receiver to send a pause frame to the sender for a specific traffic class when its buffer is congested. This prevents frame drops while maintaining separate flow control for different priorities, which is critical for converged networks carrying both storage and compute traffic. It is commonly used in RoCEv2 and Fibre Channel over Ethernet (FCoE) deployments to ensure lossless behavior.
Technical Details
PFC uses a 3-bit priority code point (PCP) field in the VLAN tag to distinguish up to eight traffic classes, each with independent pause thresholds. The pause frame specifies a duration (quantized in pause quanta) for which the sender must stop transmitting frames of that priority. Buffer sizes and pause thresholds must be tuned per the OEM spec to avoid head-of-line blocking or deadlock scenarios. PFC interacts with ETS (Enhanced Transmission Selection) to allocate bandwidth among priorities.
How Leviathan Systems Works with Priority Flow Control
In our GPU rack builds, PFC is configured on the leaf switches connecting to GPU nodes via RoCEv2 for scale-out networking, ensuring zero packet loss under bursty training traffic. We verify PFC counters and pause frame symmetry during commissioning to prevent priority starvation across the fabric.
Related service
Network Testing & Commissioning →