114 | Data Center Bridging (DCB)w w w . d e l l . c o m | s u p p o r t . d e l l . c o m Data center bridging satisfies the needs of the following types of data center traffic in a unified fabric:• LAN traffic consists of a large number of flows that are generally insensitive to latency requirements,while certain applications, such as streaming video, are more sensitive to latency. Ethernet functions asa best-effort network that may drop packets in case of network congestion. IP networks rely ontransport protocols (for example, TCP) for reliable data transmission with the associated cost of greaterprocessing overhead and performance impact.• Storage traffic based on Fibre Channel media uses the SCSI protocol for data transfer. This traffictypically consists of large data packets with a payload of 2K bytes that cannot recover from frame loss.To successfully transport storage traffic, data center Ethernet must provide no-drop service withlossless links.• Servers use InterProcess Communication (IPC) traffic within high-performance computing clusters toshare information. Server traffic is extremely sensitive to latency requirements.To ensure lossless delivery and latency-sensitive scheduling of storage and service traffic and I/Oconvergence of LAN, storage, and server traffic over a unified fabric, IEEE data center bridging adds thefollowing extensions to a classical Ethernet network:• 802.1Qbb - Priority-based Flow Control (PFC)• 802.1Qaz - Enhanced Transmission Selection (ETS)• 802.1Qau - Congestion Notification• Data Center Bridging Exchange (DCBX) protocolPriority-Based Flow ControlIn a data center network, priority-based flow control (PFC) manages large bursts of one traffic type inmultiprotocol links so that it does not affect other traffic types and no frames are lost due to congestion.When PFC detects congestion on a queue for a specified priority, it sends a pause frame for the 802.1ppriority traffic to the transmitting device. In this way, PFC ensures that large amounts of queued LANtraffic do not cause storage traffic to be dropped, and that storage traffic does not result in high latency forhigh-performance computing (HPC) traffic between servers.PFC enhances the existing 802.3x pause and 802.1p priority capabilities to enable flow control based on802.1p priorities (classes of service). Instead of stopping all traffic on a link (as performed by thetraditional Ethernet pause mechanism), PFC pauses traffic on a link according to the 802.1p priority set ona traffic type. You can create lossless flows for storage and server traffic while allowing for loss in case ofLAN traffic congestion on the same physical interface.Figure 8-1 shows how PFC handles traffic congestion by pausing the transmission of incoming traffic withdot1p priority 3.Note: In FTOS version 8.3.16.1, only the PFC, ETS, and DCBX features are supported in data centerbridging.
