BROCADE IP PRIMER

Chapter 3: Data Center Bridging (DCB)Congestion Notification802.1Qau: Congestion Notification (QCN)An end-to-end congestion management mechanism enables throttling of trafficat the end nodes in the network in the event of traffic congestion. When adevice is congested, it sends a congestion notification message to the endnode to reduce its transmission. End nodes discover when congestion easesso that they may resume transmissions at higher rates.NICNICRLBridgeCongestionNICCNMBridgeBridgeBridgePFCNICNICfrom IEEE DCB tutorialCNM (Congestion Notification Message) Message is generatedsent to ingress end station when a bridge experiences congestionRL (Rate Limiter) In response to CNM, ingress node rate-limits theflows that caused the congestionBut don’t forget that QCN is a separate protocol independent of ETS and PFC.While ETS and PFC are dependent on each other, they do not depend on orrequire QCN to function or be implemented in systems.Summary• Most data centers in large organizations use Ethernet for front-end LANslinking users to enterprise servers and Fibre Channel for back-end SANlinks between server and storage.• The DCB effort undertaken by the IEEE 802.1 work group adds new extensionsto bridging and Ethernet, so that it becomes capable of convergingLAN and storage traffic on a single link.• The combination of DCB and FCoE technologies provides a solution forthe challenges of physical infrastructure reduction and cabling simplicity• The following standards organizations are working on enhancements tothe Ethernet standards to enable DCB and FCoE:— IEEE for DCB (PFC, ETS, and DCBX)• The idea of Priority-based Flow Control (PFC) is to divide Ethernet trafficinto different streams or priorities.86 Brocade IP Primer