gute Frage 
du kannst mal mit
cmk -vvII <your-device-name>
schauen was CMK für Daten bekommt. Interessant wäre was dein Nexus für die relevanten OIDs liefert.
Nach dem Discovery sollte dann eine Zeile mit SNMP scan found kommen. Was findet CMK da bei dir
Bei einem einfachen Cisco Router sieht das bei mir so aus
SNMP scan found cisco_cpu_memory cisco_cpu_multiitem cisco_fan cisco_fantray cisco_fru_module_status cisco_fru_power cisco_fru_powerusage cisco_hsrp cisco_mem cisco_power cisco_redundancy cisco_secure cisco_temp cisco_temp_sensor cisco_temperature dell_om_disks dell_om_esmlog dell_om_fans dell_om_mem dell_om_power dell_om_processors dell_om_sensors if if64 if64name inv_cisco_bug inv_cisco_contract inv_cisco_eox inv_cisco_psirt inv_cisco_vlans inv_if pse_poe rmon_stats snmp_extended_info snmp_info snmp_os snmp_uptime ucd_cpu_util ucd_disk ucd_diskio ucd_mem ucd_processes
der nächste interessante Punkt wäre dann -> Add sections:
-> Add sections: ['cisco_cpu_memory', 'cisco_cpu_multiitem', 'cisco_fan', 'cisco_fantray', 'cisco_fru_module_status', 'cisco_fru_power', 'cisco_fru_powerusage', 'cisco_hsrp', 'cisco_mem', 'cisco_power', 'cisco_redundancy', 'cisco_secure', 'cisco_temp', 'cisco_temp_sensor', 'cisco_temperature', 'dell_om_disks', 'dell_om_esmlog', 'dell_om_fans', 'dell_om_mem', 'dell_om_power', 'dell_om_processors', 'dell_om_sensors', 'if', 'if64', 'if64name', 'inv_cisco_bug', 'inv_cisco_contract', 'inv_cisco_eox', 'inv_cisco_psirt', 'inv_cisco_vlans', 'inv_if', 'pse_poe', 'rmon_stats', 'snmp_extended_info', 'snmp_info', 'snmp_os', 'snmp_uptime', 'ucd_cpu_util', 'ucd_disk', 'ucd_diskio', 'ucd_mem', 'ucd_processes']