xus 2000 devices are not switches, but rather fabric extenders. FEXs can be thought of as external blades for Nexus switches. With a Nexus 5000, you can connect up to 12 FEXs. That's like having a 14-slot chassis when using a 5020, and the "slots" don't have to be in the same rack as the "supervisor." You can link two Nexus 5000s together, thereby making 24 FEXs available for dual-homing servers. This can be an amazingly flexible design, though it is not without its drawbacks, as we'll see in the configuration section. Newer Nexus models and versions of NX-OS support even more FEXs. Because the FEXs are not switches, they have no intelligence of their own. They don't even have console ports. To use a FEX, you must connect it to a higher-tier Nexus Chapter 18: Cisco Nexus Figure 18-6. Nexus 5010 with power supplies and fan trays removed switch like a 5000 or 7000. If you think of them like external blades for these switches, you'll understand FEXs pretty well. Beware of the original 2148 48-port 1 Gbps FEXs, because they have a couple of serious limitations. The most maddening is their inability to support EtherChannels within a single FEX, though the fact that they have Gigabit ports that don't support 100 Mbps or 10 Mbps is a close second. Cisco has phased them out and is now selling the much-im- proved 2248 FEX, which resolves these issues. Just remember that you'll need to be on NX-OS code revision 4.2(1)N1(1) or later on the 5000 to use the 2248. The 2000s also require the same rack-mount airflow tolerances mentioned in the 5000 section. A FEX mounted in an airflow-controlled rack is shown in Figure 18-7. FEXs are not, and should not be considered, switches. Their primary purpose is to connect hosts to the network. This distinction is further clarified by the nature of the interfaces on a FEX, which are described as host ports or fabric ports: Host ports Host ports are designed to connect servers (hosts) to the network. To this end, host ports have spanning tree BPDU Guard enabled, which will error-disable th