Discussion

Discussion
RMON is an SNMP specification that allows an SNMP agent (your router) to proactively
monitor its system health and performance and then send traps to an SNMP
manager. The local SNMP agent compares MIBvalues against predefined thresholds
and generates exception alarms without the need for polling by a central SNMP management
platform. This is an effective mechanism for proactive management, provided
that you have baselined and set the thresholds correctly. RMON also decreases
the amount of traffic between the manager and the router because the SNMP manager
does not always have to poll for information and it allows the manager to get
more timely status reports because the router reports events as they occur.
You can monitor many things. This recipe monitors the router’s backplane temperature.
The backplane is in the center of the router, so the temperature gives you an
idea of whether the router might be overheating. This recipe sets the threshold at 40
degrees Celsius. When this value is exceeded, an RMON event is triggered, a trap is
sent, and the event is logged.
To set up RMON, configure the OID and the threshold values that trigger the alarm
(with the set alarm commands), the router’s response to the alarm (with the set
event commands), and the NMS systems to receive the trap (with the set trap-group
commands).
The alarm’s threshold value can be an actual value, as in these two alarms (set with
the sample-type statement and absolute-value option), or the difference between the
current value and the last value (set with the delta-value option).
Finally, choose a number to identify the alarm and to link the alarm with the event.
Specify the number in the rising-alarm-index statement when monitoring a rising
threshold or in the falling-alarm-index statement when monitoring a falling threshold.
For alarm 1, rising-alarm-index 1 associates event 1 with this alarm.
The event statement hierarchy defines the router’s response to the alarm. In this recipe,
the type log-and-trap statement logs both sets of traps. The community statement
associates the events with the trap group overtemperature, which sends the
traps to the NMS system defined in the targets statement.
When you configure the trap group to handle the RMON event, the category must
be rmon-alarm. The targets are all the NMS systems to receive the trap.
Events are generated only when the threshold is first crossed in any one direction,
not after each sample period. Once the threshold is crossed, no more events are generated
until after the value crosses back into the normal range and again crosses the
threshold. This mechanism considerably reduces the quantity of alarms produced by
the router, making it easier for you to react when alarms do occur. Keep in mind that
because SNMP uses UDP, there is no guarantee of the delivery of the alarm to the
SNMP manager.
Using RMON Traps to Monitor the Router’s Temperature | 151
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