With some snmp tables, instead of setting a field to some equivalent of 'down', the entire row is removed.
Some examples of this:
OSPF neighbor state: OSPF-MIB::ospfNbrState
ISIS neighbor state: ISIS-MIB::isisISAdjState
When polling such an OID the host returns either SNMP_NOSUCHOBJECT or SNMP_NOSUCHINSTANCE. The item becomes "not supported" and it is rather difficult to write a trigger for it.
In most cases I've encountered there is a value defined for 'down', so it would be great is you could use that value and ignore the SNMP_NOSUCH* error.
I have a patch that does this by extending the OID syntax. Can't attach it to this issue: "JIRA could not attach the file as there was a missing token. Please try attaching the file again."
For static OIDs you can append an equal-sign '=' and the value you want to use
For dynamic OIDs an optional forth parameter is used as the value to used in case of SNMP_NOSUCH*