s PDU type = 4 and has to fill in enterprise, agent address, generic trap, specific trap type, time stamp fields, and variable bindings list. Figure 21.10 gives an excellent summary overview of SNMP. 21.7.3 Remote Monitoring (RMON) The tendency toward more and more distributed networks has become apparent. The wide area distribution is both geographical and logical. One method of handling this Table 21.3 SNMP Trap Codes Trap Type Value coldStart warmStart linkDown linkup authenticationFailure egpNeighborLoss enterpriseSpecific 21.7 NETWORK MANAGEMENT SYSTEMS IN ENTERPRISE NETWORKS Internet(1) 11, 3, 6, 1, ...] directory(1) mgmt(2) private(4) mib(1) Get Next system(1) interfaces(2) tcp(6) sysDesc(1) SysObjectid(2) tcpConn Table(13) tcpConnEntry(1) tcpConn tcpConnLocal tcpConnRemote State(1) Address(2) Port(5) Agent Agent Agent Agent Figure 21.10 An overview of SNMP in place. (From Figure 2-4, Ref. 6, courtesy of IEEE Press.) situation is to place remote management devices on the remote segments. These remote management devices are sometimes called probes. These probes are the remote sensors of the network management system, providing the centralized management station with the required monitoring data dealing with network operation. There is remote network monitoring (RMON) MIB that standardizes the management information sent to/from these probes. As an illustration the Ethernet RMON MIB contains nine groups. One of these groups, for example, is entitled "alarms." It compares statistical samples with preset thresholds and generating an alarm when a threshold is crossed. Another is "capture," which allows packets to be captured after they pass through a logical channel. 21.7.4 SNMP Version 2 SNMPv.2 design used the field experience gained by SNMP to sharpen and simplify the mappings to different transports. The management protocol has been separated from the transport environment, encouraging its use over practically any protocol stack. One of the weaknesses of SNMP is in the unreliable manner of handling trap mes