In the default configuration we leverage the event translator to enrich the uei.opennms.org/generic/traps/SNMP_Link_Down traps with the ifDescr, ifName and ifAlias of the associated interface in order to generate a uei.opennms.org/translator/traps/SNMP_Link_Down event. The alarm data is associated with the translated event, which leads to alarms being created with descriptive fields.
As it is currently written, the uei.opennms.org/translator/traps/SNMP_Link_Down events only get generated if we find a match for the corresponding ifIndex in the database. If none is found, then no alarm is generated.
We should update the configuration, and enrich the event translator if necessary to support translating these events for cases in which the ifIndex is not found in the database.