The Zenoss installation added its own server into the devices list as a default. However, it is having trouble reading snmp values from itself. Performing snmpwalk via the Zenoss web interface results in a timeout error.
The snmpwalk command used by Zenoss uses the IPv6 loopback address ::1. I switched this to the IPv4 equivalent 127.0.0.1 and the snmpwalk worked fine.
It turns out, the snmp agent, net-snmp, only listens to IPv4 addresses by default. Some changes need to be done to the net-snmp config file.
The following lines need to be added to /etc/snmp/snmpd.conf:
agentaddress udp:161 agentaddress udp6:161 # sec.name source community com2sec notConfigUser default public com2sec6 notConfigUser default public
Restart snmpd and it should now listen to IPv6 as well.
# service snmpd restart