e state synchronisation mechanism. ifEntry (1) GET-RESPONSE is used by the agent to provide the informa- tion requested by the manager. SET-REQUEST is used by the manager to change the value of ifPhysAddress (6) an MIB object or variable, including booting and re-booting devices. TRAP is slightly different from the above four messages, since Figure 16.23 Registration sub-tree for MIB definitions it originates from the managed element and allows them to send used in SNMP events to the manager. It is an unsolicited, asynchronous mess- age sent from the managed agent to the manager, usually to 16/22 Network management MANAGER MANAGED RESOURCE MANAGER MANAGED APPLICATION OBJECTS CENTER EVENT SNMP MANAGER SNMP AGENT Figure 16.24 Communications using SNMP indicate that threshold condition has been exceeded. Table 16.6 GET-RESPONSE <sysUpTime:TimeTicks 211930> shows the traps used within SNMP. Vendors can add their This indicates to the manager that the variable is of type Time- proprietary traps for indicating alarms raised by their equip- Ticks, having a value of 211930 or about three quarters of an hour. ment. The agent will return any protocol errors which occur, following 16.9.4 SNMP, CMOT and CMIP any of the above requests (except for TRAP) by a manager, as shown in Table 16.7 SNMP was designed initially to be an interim standard, until full As an example, suppose the manager wants to know the length of OSI based network management standards (CMIP) were available. time the agent has been running. The variable which determines this However the popularity of SNMP, with several hundred vendors is called sysUpTime and is represented by 1.3.6.1.2.1.1.3. The supporting it, means that it is likely to be in the network for many manager sends the following message to the agent: years to come. Transition to OSI could therefore be delayed and GET-REQUEST <sysUpTime> many vendors have even spoken confidently of SNMP displacing The agent replies with: TRANSPORT LAYER PROTOCOL MESSAGE NETWORK PROTOCOL DATAGRAM LAYER LOCAL LOCAL