Wikipedia
NX-OS
NX-OS is a network operating system for the Nexus-series Ethernet switches and MDS-series Fibre Channel storage area network switches made by Cisco Systems. It evolved from the Cisco operating system SAN-OS, originally developed for its MDS switches.
It is based on MontaVista Software embedded Linux and is inter-operable with other Cisco operating systems. The command-line interface of NX-OS is similar to that of Cisco IOS.