CLEAR-Flow
ExtremeWare XOS 11.3 Concepts Guide
382
See the section, “Ratio Expression Example” on page 390, for a full example of an ACL and a CLEAR-
Flow rule using a ratio expression.
Delta-Ratio Expression
A CLEAR-Flow delta-ratio expression is a combination of the delta and ratio expressions. The CLEAR-
Flow agent computes the difference from one sample to the next for each of the two counters. The ratio
of the differences is then compared to the threshold value. The following is the syntax for a CLEAR-
Flow delta-ratio expression (note the similarity to the delta expression):
delta-ratio <counterNameA> <counterNameB> REL_OPER <countThreshold> ;
min-value <min-value> ;
hysteresis <hysteresis> ;
The delta-ratio statement specifies how to compare the ratio of the counter differences with its
threshold. The difference of the sample values of
<counterNameA> is divided by the difference of the
sample values of
<counterNameB>, to compute the ratio that is compared with the <countThreshold>.
The
REL_OPER is selected from the relational operators for greater than, great than or equal to, less than,
or less than or equal to (>, >=, <, <=).
The min-value statement is optional, and sets a minimum value for the counters. If either counter is less
than the minimum value, the expression evaluates to false. If not specified, the minimum value is 1.
The
hysteresis <hysteresis> statement is optional, and sets a hysteresis value for the threshold.
After the
ratio statement is true, the value of the threshold is adjusted so that a change smaller than
the hysteresis value will not cause the statement to become false. For statements using the REL_OPER >
or >=, the hysteresis value is subtracted from the threshold; for < or <=, the hysteresis value is added to
the threshold.
For example, the following delta-ratio expression:
delta-ratio counter1 counter2 >= 5 ;
min-value 100 ;
hysteresis 1 ;
will only be true after the ratio of the deltas of the counters reached at least 5. At the time it became
true, the hysteresis value would be subtracted from the threshold (setting the threshold to 4). With the
threshold now at 4, the condition would stay true until the ratio of the deltas of the counters became
less than 4.
If the statement becomes false, the threshold is reset to its original value. You would use the hysteresis
value to prevent the rule from vacillating between the true and false states if the ratio of the deltas of
Table 53: Ratio Expression Evaluation Example
Evaluation counter1 value counter2 value ratio Rule triggered?
1 427 70 6 No
2 941 235 4 No
3 2475 412 6 Yes
4 2308 570 4 Yes
5 2313 771 3 No
6 3597 899 4 No
7 5340 1065 5 Yes
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