Building Expressions
Expressions permit you to take SNMP agent data and derive useful mathematical results.
Raw data that SNMP Management Console receives from SNMP agents can be very useful but, often it’s only the starting point. An SNMP agent on a switch may keep track of the number of data packets the switch has received, the number of packets it has discarded, and the number of packets it has passed along. However, the network administrator may be more interested in the percentage of packets discarded since this may signal a problem with the system.
1. To create a new expression, from the MIB Editor, click on Expressions, then select Mode Commands > New Expression or right-click and select New Expression.
2. From the left pane of the MIB Editor, select any MIB objects that you intend to use in the expression and drag-and-drop them on the new expression.
There may be a slight performance penalty caused by including unnecessary MIB objects in an expression. In terms of system efficiency, it’s best to add only those you need. If you find you need additional MIB objects to create your expression, you can easily add them at a later time by the same drag-and-drop method.
3. Right-click on the new expression to rename it, if desired.
4. Right-click on the renamed expression and select Edit Expression to display the Modify Expression dialog.
 
Figure 62: Edit Expression
 
The Modify Expression dialog box is, in effect, a numeric calculator, permitting the creation and modification of mathematical expressions using selected MIB objects, constants, and mathematical operations.
5. Numbers can be entered from the keyboard; mathematical functions can be entered either via the keyboard, or from the buttons of the dialog. The Insert MIB Object button can be used to insert MIB objects that have been dragged to the expression.
6. Click OK to save the edited expression.
 
Now that the new expression has been built, it can be used in a chart.