Thanks all,
I followed your advices, and able to add NAC appliance to Extreme Management Center.
But I think these articles are a bit short. For example they did not mentioned which type of authentication and privacy to select during SNMP profile creation. If we select SHA or AES, EMC can not discover the NAC appliance.
To learn which type of authentication and privacy is used by NAC, issue the "
cat /etc/snmp/snmpd.conf" command as decribed
https://gtacknowledge.extremenetworks.com/articles/Solution/NAC-controller-shows-SNMP-Contact-Lost-i...