The common mistake is to configure snmpd for v3 and forget to disable snmp v1/v2.

go to /etc/snmp/snmpd.conf and comment line with

“com2sec notConfigUser  default       public”

So full net-snmp installation and configuration should be something like:

1. yum install -y net-snmp-utils net-snmp net-snmp-libs
2. service snmpd status
3. service snmpd stop
4. net-snmp-create-v3-user -ro -A [password] -a MD5 -X snmpv3encPass -x DES snmpv3user
5. comment line with “com2sec notConfigUser  default       public”
6. service snmpd start
7. configure PRTG or other monitoring tool
8. chkconfig snmpd on
9. snmptable -v 3 -u snmpv3user -l auth  -a MD5 -A [password] localhost usmUsertable
10. snmpwalk -u snmpv3user -A [password] -a MD5 -l authnoPriv 127.0.0.1 -v3 | head