Hello,
A Fabric Interconnect is a device that is half switch half UCS (Cisco servers) Manager. In the Fabric Interconnect resides UCS Manager where you as a user give life to the real server through a Service Profile where you define stuff like the amount of vNICs/vHBAs your server will have, the VLANs/VSANs that will go through the VNICs/vHBAs, the boot order, WWPNs/WWNNs, in summary, the real virtualized server is define there.
The FEX is a device that is normally a logical extension of a parent switch which lets you take advantage of the EoR (End of the Row) and ToR (Top of the Rack) designs.
The FEX is in essence a a remote virtual line card of the parent switch, which in turn can be, for example, a Nexus 7K/5K. The FEX does not switch packets, that is done by the parent switch; even upgrades for the Fabric Extender take place only if the parent switch is upgraded.
If you are using a B series servers solution, you will need a Fabric Interconnect, which need to be connected to a type of FEX called I/O Module (IOM) which is an enhanced version of a regular Nexus 2000 (which is another type of FEX). The IOM is an enhanced version of a typical FEX cause it also manages the chassis where you would install the blade servers (known as B series servers)
As you can see, their description and functionality is totally different, so I hope I made my point, but let me know if you would like me to expand a lit but on their definition, use and capabilities.
Regards,
-Kenny
