Enterasys Networks 700 - Manual (Page 143)

How Rising and Falling Thresholds Work 4-27
Alarm Configuration
The top portion of the window contains the device information boxes, as well as
the event index number and the event description; the log itself includes the
following fields:
Index This uniquely identifies this occurrence of the event.
Time Indicates the date and time of each event occurrence.
Description Provides a detailed description of the alarm that
triggered the event: whether it was a rising or falling
alarm, the alarm index number, the alarm variable name
and object identifier (OID), the alarmSampleType
(1=absolute value; 2=delta value), the value that
triggered the alarm, the configured threshold that was
crossed, and the event description. Use the scroll bar at
the bottom of the log to view all the information
provided.
Each log will hold only a finite number of entries, which is determined by the
resources available on the device; when the log is full, the oldest entries will be
replaced by new ones.
How Rising and Falling Thresholds Work
Rising and falling thresholds are intended to be used in pairs, and can be used to
provide notification of spikes or drops in a monitored value — either of which can
indicate a network problem. To make the best use of this powerful feature, pairs
of thresholds should not be set too far apart, or the alarm notification process may
be defeated: a built-in hysteresis function designed to limit the generation of
events specifies that, once a configured threshold is met or crossed in one
direction, no additional events will be generated until the opposite threshold is
met or crossed. Therefore, if your threshold pair spans a wide range of values,
and network performance is unstable around either threshold, you will only
receive one event in response to what may be several dramatic changes in value.
To monitor both ends of a wide range of values, set up two pairs of thresholds:
one set at the top end of the range, and one at the bottom.
TIP
The current version of the Basic Alarms window only allows you to configure a single pair
of thresholds for each alarm variable on each interface; be sure to keep this hysteresis
function in mind when configuring those threshold values.