1. A method of configuring a policy used by a network device by specifying an action to be taken in the event an element of the policy is undefined, the method comprising: (a) providing a configuration interface for configuring a policy of a network device; (b) identifying, by the configuration interface, a policy comprising a first action to be taken based on an evaluation of an expression; (c) receiving, via the configuration interface, information identifying a second action for the policy, the second action to be taken if an element of the policy is undefined.