276 CHAPTER 7: MONITORING THE NETWORKFor details on how to start monitoring an unmonitored device or link, orto stop monitoring a monitored device or link, see “Starting and StoppingMonitoring” on page 293.For more information on event severities see “Event Severities” onpage 321Monitoring State and the Grouped ViewWhen you are viewing the devices in your network grouped by subnet,3Com Network Director propagates the monitoring states of devices,links and submaps up the submap hierarchy.Each submap is colored toindicate the most critical monitoring state of the devices, links orsubmaps that it contains.For example, if a subnet contains two devices, one of which has a redmonitoring state and one of which has a yellow monitoring state, thesubnet will be colored red. If the event causing the red device'smonitoring state is subsequently resolved, the subnet will change to becolored yellow, as this is now the most critical monitoring state containedwithin the subnet.When determining the monitoring state of a submap, 3Com NetworkDirector ignores any map items or links that are visible at a higher level inthe hierarchy than the submap itself.For example, the monitoring state ofa router will not affect the monitoring state of the subnets that it ispresent on, as the router is visible in the top-level map, which is thehighest level in the hierarchy.MonitoringTechniques3Com Network Director uses various techniques to monitor the state ofdevices and links within your network.The techniques used for aparticular monitor depend upon the feature that is being monitored, andcan be broadly broken down into active and passive monitoring:Active Monitoring: Active monitoring works by 3Com NetworkDirector polling, and possibly retrieving data from, a device on a regularbasis.Once the device has been polled, or once the data has beenretrieved from the device, 3Com Network Director can analyze the resultsto determine the state of the device.There are various mechanisms usedfor active monitoring, including sending IP pings, retrieving data fromSNMP MIBs, and directly checking for service availability.
