ModSnmp Manual v3.09 - 3.3. Engine ID
The Engine ID in the SNMP page is used in SNMPv3 to uniquely identify the SNMP agent. It is also known as the "Agent ID". The default value is automatically generated from the IP address of the machine that ModSnmp is running on. Do not change the engine ID unless you know what you are doing.
To change the engine ID, enter the new ID in the Engine ID field as hex bytes and click Apply.
WARNING: The engine ID is used in the generation of authentication and encryption keys. If you change the engine ID, you will have to re-enter all "secure" users, i.e. users that have authentication pass-phrases.