This document describes priority flow control (PFC), one of the functions of IEEE 802.1 Data Center Bridging, introduced to address reliability at Layer 2 on the Ethernet medium. PFC enables lossless semantics for a subset of the Layer 2 flows carried on an Ethernet segment. The subset of flows subject to the lossless semantics is identified by the IEEE 802.1p class of service (CoS) information found in the IEEE 802.1Q tags. PFC is currently being defined in the IEEE 802.1Qbb workgroup. Although the Cisco® proposal for PFC has not achieved standards status yet, a significant number of vendors have embraced the frame format proposed by Cisco, and therefore PFC implementations can be deployed today without risk of future incompatibility in that respect. PFC is an important requirement for enabling Fibre Channel over Ethernet (FCoE) for storage consolidation, and this document describes how the Cisco Nexus™ 5000 Series Switches use PFC to build a lossless Layer 2 medium for FCoE.
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