Step 5: Set up a Threshold

A threshold defines limits against the network performance metrics (CPU load, bandwidth, disk space, and so on) of a managed entity to trigger an event (record) when a datasource value goes above or below the specified limit. The threshold types are as follows:

High: Equals or exceeds the threshold value, and re-arms when it equals or drops below the re-arm value (for example, bandwidth use exceeds the specified limit).
Low: Equals or drops below the threshold value and re-arms when it equals or comes back up above the re-arm value (for example, available disk space falls under the specified value).
Absolute change: Changes by the specified amount (for example, on a fiber-optic link, a change in loss of anything greater than 3 dB is a problem regardless of what the original or final value is).
Relative change: Changes by a specified percentage (for example, available disk space changes by more than 5% from the last poll).

These thresholds can be basic (tested against a single value) or expression-based (evaluated against multiple values in an expression). This Quick Start guide shows how to create both kinds of threshold.

Create a basic threshold

This procedure describes how to create a basic threshold for a single, system-wide variable: the number of logged-in users. This threshold tells Meridian to create an event when the number of logged-in users on the device exceeds two, and re-arm when it falls below two.

Click the gear symbol in the top-right of the screen.
Under Performance Measurement, click Configure Thresholds. A list of preconfigured threshold groups appears.

For information on how to create a threshold group, see Create a Threshold Group in the Deep Dive section.
Click Edit beside the netsnmp group.
Click Create New Threshold at the bottom of the Basic Thresholds area.

Set the following parameters and click Save:

Basic threshold configuration settings
Field	Description	Input value
Type	Triggers an event when the datasource value equals or exceeds the threshold value, and re-arms when it equals or drops below the re-arm value.	high
Datasource	Name of the datasource that you want to threshold against. For this tutorial, we have provided the datasource for logged-in users. For information on how to determine a metric’s datasource, see Determine the datasource in the Deep Dive section.	hrSystemNumUsers
Datasource label	Optional text label.	Leave blank.
Value	The value above which you want to trigger an event. In this case, we want to trigger an event when the number of logged-in users exceeds two.	2
Re-arm	The value below which we want the system to re-arm. In this case, we want to re-arm the system when the number of logged-in users falls below two.	2
Trigger	The number of consecutive times that the threshold value can occur before the system triggers an event. Since our default polling period is 5 minutes, a value of 3 means Meridian will create a threshold event if there are more than two users logged in for 15 minutes.	3
Description	Optional text to describe your threshold.	Two users
Triggered UEI	A custom unique event identifier (UEI) sent into the events system when the threshold is triggered. Defining a custom UEI for each threshold makes it easier to create notifications. If left blank, it defaults to the standard thresholding UEIs.	Leave blank.
Re-armed UEI	A custom unique event identifier (UEI) sent into the events system when the threshold is re-armed.	Leave blank.

Test the threshold

To test the threshold that you just created, follow these steps:

SSH to the monitored node and log in to it using two different user accounts.
In the main menu, choose Status Events.
Under Event Queries, click All Events. You should see an event that indicates that your threshold triggered when more than one user logged in.
Log one user out of the node. The Events List page should indicate that the system has re-armed.

Expression-based thresholds

By default, Meridian includes two expression-based thresholds. This section explains the threshold for CPU use, where Meridian will create an event when the five-minute CPU load average metric equals or exceeds 70% for two consecutive measurement intervals.

Expression-based thresholds are useful when you need to threshold on a percentage, not the actual value of the data collected.

Expression-based thresholds work only if the datasources in question are in the same directory.

To view the CPU usage threshold, follow these steps:

Click the gear symbol in the top-right of the screen.
Under Performance Measurement, click Configure Thresholds.
Click Edit beside the netsnmp group.
In the Expression-Based Thresholds area, view the first threshold (with the description "Trigger an alert when five …").

The following table describes the parameters and their suggested configurations:

Basic expression-based threshold configuration settings
Parameter	Description	Value
Type	Triggers an event when the datasource value equals or exceeds the threshold value, and re-arms when it equals or drops below the re-arm value.	high
Expression	Divides the five-minute CPU load average by 100 (to obtain the effective load average¹), which is then divided by the number of CPUs. The resulting value is multiplied by 100 to calculate a percentage.	((loadavg5 / 100) / CpuNumCpus) * 100
Datasource type	The type of datasource from which you are collecting data.	node
Datasource label	Optional text label.	Leave blank.
Value	Threshold value. Triggers an event when the five-minute CPU load average equals or exceeds the defined value.	70
Re-arm	Re-arm value. Re-arms the system when the five-minute CPU load average equals or drops below the defined value.	50
Trigger	The number of consecutive times that the threshold value can occur before the system triggers an event. In this case, it triggers an event when the five-minute CPU load average goes above 70% for two consecutive polling periods.	2
Description	Optional text to describe your threshold.	Trigger an alert when the five-minute CPU load average metric reaches or goes above 70% for two consecutive measurement intervals.
Triggered UEI	A custom unique event identifier (UEI) sent into the events system when the threshold is triggered. Defining a custom UEI for each threshold makes it easier to create notifications. If left blank, it defaults to the standard thresholding UEIs.	Leave blank.
Re-armed UEI	A custom unique event identifier (UEI) that is sent into the events system when the threshold is re-armed.	Leave blank.

SNMP does not report in decimals, which is why the expression divides the loadavg5 by 100.

Beyond Quick Start

Refer to Thresholding in the Deep Dive section for more information on thresholding, including the following: