Hello,
The trap is the following :
conf t :
snmp-server enable trap | linkup | linkdown |
And the MIB for this is the IF-MIB, you can find the respective information about the trap and the OIDs in the following links :
IF-MIB Link Operational Status for Subinterfaces
http://www.cisco.com/c/en/us/td/docs/ios/12_0s/feature/guide/s_ifmib.html
Cisco IOS SNMP Traps Supported and How to Configure Them
http://www.cisco.com/c/en/us/support/docs/ip/simple-network-management-protocol-snmp/13506-snmp-traps.html
Hope it helps!
Claudio Gonzalez S