Answer
For Ethernet and ATM types of interfaces, Junos OS does not send link down traps for a logical interface if the physical interface is down to prevent flooding alarms for the same root cause. However, when the physical interface and logical interfaces come back up, traps are sent indicating link up. This is because the physical interface coming up does not necessarily mean the logical interfaces are also coming up.
For SONET types of interfaces with PPP encapsulation, Junos OS does send link down traps for a logical interface if the physical interface is down. When the physical interface and logical interfaces come back up, traps are sent for both the physical and logical interfaces indicating link up.
For SONET types of interfaces with HDLC encapsulation, Junos OS does not send link down traps for a logical interface if the physical interface is down. When the physical interface and logical interfaces come back up, traps are sent for both the physical and logical interfaces indicating link up.
For channelize interfaces with PPP encapsulation, Junos OS does send link down traps for a logical interface if the physical interface is down. When the physical interface and logical interfaces come back up, traps are sent for both the physical and logical interfaces indicating link up.
For channelize interfaces with HDLC encapsulation, Junos OS does not send link down traps for a logical interface if the physical interface is down. When the physical interface and logical interfaces come back up, traps are sent for both the physical and logical interfaces indicating link up.
For more information, click SNMP MIBs and Traps.