Extreme Networks ExtremeWare XOS Guide Manual page 381

Table 52: Delta Expression Evaluation Example (Continued)
Evaluation
10
See the section, "Delta Expression Example" on page
Flow rule using a delta expression.
Ratio Expression
A CLEAR-Flow ratio expression compares the ratio of two counter values with the threshold value. The
following is the syntax for a CLEAR-Flow ratio expression:
ratio <counterNameA> <counterNameB> REL_OPER <countThreshold> ;
min-value <min-value> ;
hysteresis <hysteresis> ;
The statement specifies how to compare the ratio of two counters with its threshold. The value of
ratio
is divided by the value of
<counterNameA>
compared with the
<countThreshold>
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>
After the statement is true, the value of the threshold is adjusted so that a change smaller than
ratio
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 ratio expression:
ratio counter1 counter2 >= 5 ;
min-value 100;
hysteresis 1 ;
will only be true after the ratio of the counters reached at least 5 and the counter values are at least 100.
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
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 between the
counter values is near the threshold. If the hysteresis value is greater than the threshold value, the
hysteresis value will be set to zero.
In Table 53 is an example of evaluating the CLEAR-Flow ratio expression above multiple times. Notice
that the rule is not triggered at the first evaluation because both counters have not yet reached the min-
value of 100. The rule first triggers at evaluation 3, when ratio of the two counters exceeds 5. Once the
rule is triggered, it remains triggered until the ratio value is less than 4 (the original threshold minus the
hysteresis), at evaluation 5. At evaluation 7, the rule is again triggered when the ratio reaches 5. The
rule will remain triggered until the ratio drops below 4.
ExtremeWare XOS 11.3 Concepts Guide
counter1 value
1230
<counterNameB>
. The
REL_OPER
statement is optional, and sets a hysteresis value for the threshold.
Adding CLEAR-Flow Rules to ACLs
Delta value
99
389, for a full example of an ACL and a CLEAR-
, to compute the ratio. That ratio is
is selected from the relational operators for
Rule triggered?
Yes
381