Which message signals when critical events take place in SNMP?

In Simple Network Management Protocol (SNMP), the message that signals when critical events take place is the Trap message. Traps are asynchronous notifications sent from an SNMP agent to an SNMP manager. They are used to alert the manager about significant events, such as device failures, breaches in threshold limits, or other notable changes in the managed elements.

When a critical event occurs, the SNMP agent generates a trap message that is sent immediately to the configured SNMP manager without the manager having to request it first. This real-time alerting mechanism is essential for effective network management, as it allows administrators to respond promptly to issues as they arise.

The other message types serve different functions:

• GetRequest is used by the manager to request information from the agent.

• SetRequest is used by the manager to modify configurations or set values on the agent.

• Response messages are sent back by the agent to acknowledge the receipt of a GetRequest or SetRequest, providing the requested information or confirmation of a change.

Thus, while GetRequest, SetRequest, and Response messages are integral to SNMP's communication, it is the Trap message that specifically indicates critical events, making it the correct choice in this context.