Hi Marcelo,
the snmp-server enable traps command enables just all types of traps that the IOS version supports.
The message apperes because of this two, which are mutually exclusive:
R1(config)#snmp-server enable traps ospf cisco-specific state-change shamlink interface
R1(config)#snmp-server enable traps ospf cisco-specific state-change shamlink interface-old
% Cannot enable both sham-link state-change interface traps.
% Deprecated sham link interface trap not enabled.
It's recommended to only enable the traps you really need.
Informs were introduced with SNMPv2, and they have the same format and purpose as traps.
The main difference is that traps are send in a hit-or-miss fashion whereas informs expect an acknowledge and will be re-send if unacknowledged.
Hope that helps
Rolf