the USM (snmpv3) does have following construction:
Configure snmp-user credentials:
#configure snmpv3 add user snmpuser authentication md5 authcred privacy privcred
Add snmp-user to the group:
#configure snmpv3 add group RW user snmpuser sec-model usm
define what the group can do:
#configure snmpv3 ad access RW sec-model usm sec-level priv read-view ViewAll write-view ViewAll notify-view ViewAll
the "ViewAll" can be defined:
#configure snmpv3 add mib-view ViewAll subtree 1.0.0.0/0
in your case:
#configure snmpv3 ad access RW sec-model usm sec-level priv notify-view ViewPortUpDown
#configure snmpv3 add mib-view ViewAll subtree xxxxxx
check what is the OID for the port-up/port-down and replace the xxxxx with your OIDs
the next step will be to configure the snmptrap / inform to be sent with these credentials
Regards
Zdeněk Pala