362 CHAPTER 34: RMON CONFIGURATIONmanaged network devices are connected. Thus, the NMS can further manage thenetworks.Commonly Used RMONGroupsEvent groupEvent group is used to define the indexes of events and the processing methods ofthe events. The events defined in an event group are mainly used by entries in thealarm group and extended alarm group to trigger alarms.You can specify a network device to act in one of the following ways in responseto an event:■ Logging the event■ Sending trap messages to the NMS■ Logging the event and sending trap messages to the NMS■ No processingAlarm groupRMON alarm management enables monitoring on specific alarm variables (such asthe statistics of a port). When the value of a monitored variable exceeds thethreshold, an alarm event is generated, which then triggers the network device toact in the way defined in the events. Events are defined in event groups.With an alarm entry defined in an alarm group, a network device performs thefollowing operations accordingly:■ Sampling the defined alarm variables periodically■ Comparing the samples with the threshold and triggering the correspondingevents if the former exceed the latterExtended alarm groupWith extended alarm entry, you can perform operations on the samples of alarmvariables and then compare the operation results with the thresholds, thusimplement more flexible alarm functions.With an extended alarm entry defined in an extended alarm group, the networkdevices perform the following operations accordingly:■ Sampling the alarm variables referenced in the defined extended alarmexpressions periodically■ Performing operations on the samples according to the defined expressions■ Comparing the operation results with the thresholds and triggeringcorresponding events if the operation result exceeds the thresholds.History groupAfter a history group is configured, the Ethernet switch collects network statisticsinformation periodically and stores the statistics information temporarily for lateruse. A history group can provide the history data of the statistics on networksegment traffic, error packets, broadcast packets, and bandwidth utilization.
