[root@vps libexec]# ./check_snmp -H 121.XXX.XX.11 -C
public -o
1.3.6.1.4.1.2021.9.1.2.1
SNMP problem - No data received from host
CMD: /usr/bin/snmpget -t 1 -r 5 -m '' -v 1 [authpriv] 121.XX.XX.XX:161
1.3.6.1.4.1.2021.9.1.2.1
Is snmpd(the agent) running on the host you are querying.
Did you set
the read only community(hint set it to something other than public)
string in the host where your agent is running.
http://en.wikipedia.org/wiki/Simple_Network_Management_Protocol seems
to be a good place to start looking for information.
-Tarun