How a Policy Expression Is Evaluated

During evaluation, the policy framework software converts policy actions to values of TRUE or FALSE, which are factors in determining the flow control action that is performed upon a route. However, the software does not actually perform a flow control action on a route until it evaluates an entire policy expression.

The policy framework software evaluates a policy expression as follows:

1. The software evaluates a route against the first routing policy in a policy expression and converts the specified or default action to a value of TRUE or FALSE. (For information about the policy action conversion values, see Table 17.)
2. The software takes the value of TRUE or FALSE and evaluates it against the logical operator used in the policy expression (see Table 18). Based upon the logical operator used, the software determines whether or not to evaluate the next routing policy, if one is present.

   The policy framework software uses a method of shortcut evaluation. When a result is certain, the software stops evaluating subsequent routing policies in the policy expression. For example, if the policy expression specifies logical AND and the evaluation of the first routing policy returns the value of FALSE, the software determines that the output will be FALSE no matter what the values of the unevaluated routing policies are. Therefore, the software does not evaluate the subsequent routing policies in this policy expression.

3. The software performs Step 1 and Step 2 for each subsequent routing policy in the policy expression, if they are present and if the software has determined that it is appropriate to evaluate them.
4. After evaluating the last routing policy, if it is appropriate, the software evaluates the value of TRUE or FALSE obtained from each routing policy evaluation. Based upon the logical operator used, it calculates an output of TRUE or FALSE.
5. The software converts the output of TRUE or FALSE back to an action. (For information about the policy action conversion values, see Table 17.) The action is performed.

   If each policy in the expression returned a value of TRUE, the software converts the output of TRUE back to the flow control action specified in the last policy. For example, if the policy expression (policy1 && policy2) is specified and policy1 specifies accept and policy2 specifies next term, the next term action is performed.

   If an action specified in one of the policies manipulates a route characteristic, the policy framework software carries the new route characteristic forward during the evaluation of the remaining policies. For example, if the action specified in the first policy of a policy expression sets a route’s metric to 500, this route matches the criteria of metric 500 defined in the next policy. However, if a route characteristic manipulation action is specified in a policy located in the middle or the end of a policy expression, it is possible, because of the shortcut evaluation, that the policy is never evaluated and the manipulation of the route characteristic never occurs.

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