Consider how a linkUp trap might be described:
linkUp NOTIFICATION-TYPE OBJECTS { ifIndex } STATUS current DESCRIPTION "A linkUp trap signifies that the SNMPv2 entity, acting in an agent role, recognizes that one of the communication links represented in its configuration has come up." ::= { snmpTraps 4 }
According to this invocation, the trap authoritatively identified as
{ snmpTraps 4 }
is used to report a link coming up.