Atthe fabric interconnect, the server-facing Ethernet port receives the Ethernet and Fibre Channel traffic. The
fabricinterconnect (using Ethertype to differentiate the frames) separates the two traffic types. Ethernet frames
andFibre Channelframes are switchedto theirrespective uplink interfaces.
Fibre Channel over Ethernet
CiscoUCS leverages Fibre Channelover Ethernet (FCoE) standard protocolto deliver Fibre Channel.The
upperFibre Channellayers are unchanged, sothe FibreChannel operational model is maintained.FCoE
networkmanagement and configuration is similar to a native Fibre Channel network.
FCoEencapsulates Fibre Channel traffic over a physical Ethernet link. FCoE is encapsulated over Ethernet
withthe use of a dedicated Ethertype, 0x8906, so that FCoE traffic and standard Ethernet traffic can be carried
onthe same link. FCoE has been standardized by the ANSI T11 Standards Committee.
FibreChannel traffic requires a lossless transport layer. Instead of the buffer-to-buffer credit system used by
nativeFibre Channel,FCoE depends uponthe Ethernetlink to implementlossless service.
Ethernetlinks on the fabric interconnect provide two mechanisms to ensure lossless transport for FCoE traffic:
• Link-level flow control
• Priority flow control

Link-Level Flow Control

IEEE802.3x link-level flow control allows a congested receiver to signal the endpoint to pause data transmission
fora short time. This link-level flow control pauses all traffic on the link.
Thetransmit and receive directions are separately configurable. By default, link-level flow control is disabled
forboth directions.
Oneach Ethernet interface, the fabric interconnect can enable either priority flow control or link-level flow
control(but not both).

Priority Flow Control

Thepriority flow control (PFC) feature applies pause functionality to specific classes of traffic on the Ethernet
link.For example, PFC can provide lossless service for the FCoE traffic, and best-effort service for the standard
Ethernettraffic. PFC can provide different levels of service to specific classes of Ethernet traffic (using IEEE
802.1ptraffic classes).
PFCdecides whether to apply pause based on the IEEE 802.1p CoS value. When the fabric interconnect
enablesPFC, it configures the connected adapter to apply the pause functionality to packets with specific CoS
values.
Bydefault, the fabric interconnect negotiates to enable the PFC capability. If the negotiation succeeds, PFC
isenabled andlink-level flow controlremains disabled (regardless ofits configurationsettings). If thePFC
negotiationfails, you can either force PFC to be enabled on the interface or you can enable IEEE 802.x
link-levelflow control.
Cisco UCS Manager GUI Configuration Guide, Release 2.0
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Unified Fabric