R&M Data Center Handbook

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3.8.4. Fibre Channel over Ethernet (FCoE)
FCoE is a protocol for transmitting Fibre Channel frames in networks based on full-duplex Ethernet. The essential
goal in introducing FCoE is to promote I/O consolidation that is based on Ethernet (IEEE 802.3), with an eye to
reducing the physical complexity of network structures, especially in data centers.
FCoE makes it possible to use one standard physical infrastructure for both the transmission of Fibre Channel as
well as conventional Ethernet. The essential advantages of this technology include scalability and the high
bandwidths of Ethernet-based network, commonly 10 Gbit/s at the present time.
On the other hand, the use of Ethernet for the transport of Fibre Channel frames also brings to bear the disadvantages
of the classic Ethernet protocol, such as frame loss in overload situations. Therefore some improvements
to the standard are necessary in order to ensure reliable Ethernet-based transmission. These enhancements
are being driven forward with Data Center Bridging (see section 3.8.7).
A further definite advantage can be seen in the current virtualization strategy that prevails for many data center
providers, since FCoE in the end also represents, in practice, a form of virtualization technology that is based on
physical media, and as such can partially be applied in host systems for virtualized servers.
Consolidation strategies of this nature can therefore …
… reduce expenditures and costs for a physical infrastructure that consists of network elements and cables.
… reduce the number and total costs of the network interface cards required in terminal devices like servers.
… reduce operating costs (power supply and heat dissipation).
Converged 10 GbE
Converged 10 GbE is a standard for networks which merge 10 Gbit/s Ethernet and 10 Gbit/s Fibre Channel. Fibre
Channel over Ethernet (FCoE) also falls under this convergence approach. The FC packets are encapsulated in
the header of the Ethernet frame, which then allows the Converged Ethernet topology to be used.
If switches are provided with FCoE
wherever possible (because of the
different packet sizes), they are
then transparent for FC and iSCSI
storage and for LAN.
FC encapsulation
Fibre Fiber Channel payload
FC
header
header
FC
header
FCoE
FCoE
header
header
Ethernet
header
Converged Network Adapter
(CNA)
Ethernet data:
1 / 10 Gbps
Server
10GBase-T: Data & FCoE
FC Storage:
1 / 2 / 4 / 8 Gbps
Converged Network Switch
FC
10GBase-T
A typical Fibre Channel data frame has a payload of 2,112 bytes (data), a header, and a CRC field (checksum
field). A conventional Ethernet frame has a maximum size of 1,518 bytes. In order to achieve good performance,
one must avoid having to break an FCoE frame into two Ethernet frames and then put them back together. One
must therefore use a jumbo frame or at least a “baby jumbo”, that is not (yet) standardized, but offered by various
manufacturers anyway.
The 3-stage migration path from Fibre Channel to Fibre Channel over Ethernet per the Fibre Channel Industry
Association (FCIA) was already shown in section 3.7.2.
An essential requirement of the FCoE-supported Ethernet environment is that it be “lossless”, which means that
no packets may be thrown away. Manufacturers implement “lossless” Ethernet by registering a possible buffer
overflow and stopping the transmission afterwards. This is because in general, Ethernet frames are lost only in
case of a buffer overflow.
Page 104 of 156 © 08/2011 Reichle & De-Massari AG R&M Data Center Handbook V2.0