 instance) for which a value is being communicated. A trap is an unsolicited packet sent from an agent to a manager after sensing a pre- specified condition such as a cold start, link down, authentication failure, or other such event. Agents always receive SNMP requests on UDP port 161, and network management consoles always receive traps on UDP port 162. This requirement means that multiple applications on the same management station that wish to receive traps must usually pass control of this port to an intermediate software process. This process receives traps and routes them to the appropriate application. Figure 21.9 illustrates the SNMP trap PDU structure. We must appreciate that the Trap PDU structure differs from the structure of the other four PDUs shown in Figure 21.8. Like the other PDU structure, the first field, PDU type, will be in this case PDU type = 4 (see Table 21.1). The next field is called the Enterprise Field and identifies the management enterprise under whose registration authority the trap was defined. As an example, the OID prefix {1.3.6.1.4.123} would identify Newbridge Networks Corporation as the enterprise sending the trap. Further identification is provided in the Agent Address field, which contains the IP address of the agent. In some circumstances a non-IP transport protocol is used. In this case the value 0.0.0.0 is returned. After the Agent Address field in Figure 21.9, we find the Generic Trap Type which provides more specific information on the event being reported. There are seven defined values (enumerated INTEGER types) for this field as shown in Table 21.3. The next field is the Time Stamp field that contains the value of the sysUpTime object. This represents the amount of time elapsed between the last re-initialization of the agent in question and the regeneration of the trap. As shown in Figure 21.8, the last field contains the variable bindings. A trap is generated by an agent to alert the manager that a predefined event has occurred. To generate a trap, the agent assign