Understanding FCoE: Frame Structure, Components, and Comparison with iSCSI and Fibre Channel

Previously we discussed Enhanced Ethernet and its application in FCoE. The iSCSI protocol uses TCP/IP over standard Ethernet, making it suitable for data transfer in small‑to‑medium organizations without requiring Fibre Channel expertise.

At the early stage of FCoE development, iSCSI could not compete with FC for enterprise‑grade storage for two main reasons. First, most iSCSI LANs operated on slower Gigabit Ethernet, while FC already offered 4 Gb and 8 Gb speeds; the TCP/IP stack added processing overhead, latency and reduced throughput.

Second, FC storage LANs benefited from mature vendor‑supported deployment and management tools, whereas iSCSI lagged. Today, advances in Ethernet and the rapid development of SDN have largely removed these limitations, giving Ethernet higher throughput and a richer ecosystem than FC.

FCoE Frame Structure : An FCoE frame is an Ethernet frame that carries an FCoE PDU (Protocol Data Unit). The frame format follows IEEE 802.3‑2008, and the MAC data field contains the FCoE PDU.

In FCoE networking, the terms CAN (Converged Network Adapter) and CNS (Converged Network Switch) refer to Ethernet adapters and switches that support both FC and IP traffic. A CAN combines an FC adapter and an Ethernet adapter, while a CNA provides both NIC and FCoE driver functions on a single board.

CNS (Converged Network Switch) functions like an FC switch but uses Ethernet as the physical layer, offering FC ports as FCoE Forwarders (FCF) that translate between FCoE and FC frames.

The FCoE I/O forwarding process works as follows: a CAN card encapsulates FC frames into FCoE frames inside Ethernet frames and sends them via MAC. When the frames reach an FCoE switch (FCF), the switch extracts the FC payload and forwards it to the FC fabric, which then delivers the data to the storage device.

FCF devices are relatively expensive, so they are usually not connected directly to servers; instead, an FSB (FIP Snooping Bridge) is placed between the server and the FCF to establish a virtual link and improve security.

Classification of FCoE Switches

FCoE switches are categorized by function into FCF, NPV, FDF, and FSB.

FSB (FIP Snooping Bridge) : Listens to FIP messages without participating in FIP control, controls link access based on those messages, and forwards traffic via MAC. It provides 10 GE FCoE access with Ethernet pass‑through but cannot form an independent network and requires traffic to pass through an FCF.

NPV (N_Port Virtualization) : Acts as an N_Port proxy that participates in FIP control, forwards traffic using FCID, and provides FCoE access while supporting traditional FC ports for integration with existing FC fabrics. It cannot form an independent network and relies on an FCF, leading to potential traffic looping.

NPV enables a single HBA (N_Port) to register multiple WWPNs and N_Port IDs, allowing multiple virtual machines on a host to have independent identifiers for SAN zoning and LUN allocation, provided the switch also supports NPIV.

FCF (FC Forwarder) : Provides FIP registration services, name services (NS), RSCN, and FSPF on the control plane, and forwards traffic using FCID on the data plane. It can interoperate with traditional FC networks and can form an independent fabric, though its scale is limited by Domain ID specifications.

The FCF contains a Fibre Channel switching element that includes at least one lossless Ethernet MAC (FCF‑MAC) paired with an FCoE controller and a lossless Ethernet bridge. It provides E_Port and F_Port interfaces to connect Ethernet and FC sides.

FDF (FCoE Data Forwarder) : Participates in the FIP protocol on the control plane but does not provide fabric services. It forwards traffic using FCID and supports local forwarding, allowing integration with traditional FC networks without traffic looping, though it cannot form an independent network.

In summary, FCoE combines the benefits of FC and iSCSI over Ethernet, and modern Ethernet/SDN technologies have largely overcome the early performance and management disadvantages of iSCSI, while various FCoE switch types (FCF, NPV, FDF, FSB) offer different trade‑offs between functionality, cost, and network topology.