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Understanding RMON

RMON Overview

The Junos OS supports the Remote Network Monitoring (RMON) MIB (RFC 2819), which allows a management device to monitor the values of MIB objects, or variables, against configured thresholds. When the value of a variable crosses a threshold, an alarm and its corresponding event are generated. The event can be logged and can generate an SNMP trap.

An operational support system (OSS) or a fault-monitoring system can be used to automatically monitor events that track many different metrics, including performance, availability, faults, and environmental data. For example, an administrator might want to know when the internal temperature of a chassis has risen above a configured threshold, which might indicate that a chassis fan tray is faulty, the chassis air flow is impeded, or the facility cooling system in the vicinity of the chassis is not operating normally.

The RMON MIB also defines tables that store various statistics for Ethernet interfaces, including the etherStatsTable and the etherHistoryTable. The etherStatsTable contains cumulative real-time statistics for Ethernet interfaces, such as the number of unicast, multicast, and broadcast packets received on an interface. The etherHistoryTable maintains a historical sample of statistics for Ethernet interfaces. The control of the etherHistoryTable, including the interfaces to track and the sampling interval, is defined by the RMON historyControlTable.

1. Configure SNMP, including trap groups. You configure SNMP at the [edit snmp] hierarchy level.
2. Configure rising and falling events in the eventTable, including the event types and trap groups. You can also configure events using the CLI at the [edit snmp rmon event] hierarchy level.
3. Configure alarms in the alarmTable, including the variables to monitor, rising and falling thresholds, the sampling types and intervals, and the corresponding events to generate when alarms occur. You can also configure alarms using the CLI at the [edit snmp rmon alarm] hierarchy level.

   Extensions to the alarmTable are defined in the Juniper Networks enterprise-specific MIB jnxRmon (mib-jnx-rmon.txt).

Alarm Thresholds and Events

By setting a rising and a falling threshold for a monitored variable, you can be alerted whenever the value of the variable falls outside the allowable operational range (see Figure 1).

Figure 1: Setting Thresholds

Events are only generated when the alarm threshold is first crossed in any one direction rather than after each sample interval. For example, if a rising threshold alarm, along with its corresponding event, is raised, no more threshold crossing events occur until a corresponding falling alarm occurs. This considerably reduces the quantity of events that are produced by the system, making it easier for operations staff to react when events do occur.

Before you configure remote monitoring, you should identify what variables need to be monitored and their allowable operational range. This requires some period of baselining to determine the allowable operational ranges. An initial baseline period of at least
3 months is not unusual when you first identify the operational ranges and define thresholds, but baseline monitoring should continue over the life span of each monitored variable.