74	ROUTING POLICY

action. Then the policy evaluation is terminated. However, some implementations add the facility to permit subsequent policy elements to be evaluated even after a match has already been found. This can produce exceptionally complex and powerful policy, but with gains in complexity and power comes greater difficulty of understanding. It is, therefore, vital that the implementer of the policy uses whatever means available to ensure the clarity of purpose of his policy.

6.4 POLICY MATCHES

Routing prefixes can be matched on a range of criteria, some of which are specific to individual prefixes (e.g., prefix length, route metric, originating protocol) and some of which are specific to particular routing protocols (e.g., BGP community, IS-IS level). For complete lists of all policy match conditions, refer to the command references of your chosen vendors’ software. It is these match criteria that are the basis upon which you can identify the packets or prefixes to which you are going to apply the policy action.

6.5 POLICY ACTIONS

As with policy matches, there is a vast array of policy actions, many of which are specific either to packets or to a particular protocol to which a policy is being applied (e.g. set the BGP community, set the OSPF external type). For complete lists of all policy actions, refer to the command references of your chosen vendors’ software.

6.5.1 THE DEFAULT ACTION

The default action is another area in which different vendors have chosen different solutions. Indeed, vendors have made different choices for different elements of their policy tool kits. In all cases, however, the default policy action is either permit or deny. This sounds like an obvious statement, but it is important to realize that the default action is nothing more than one of these two actions. It is essential for the successful implementation of your policy that you know the default action for a given vendor and a given policy element.

6.5.2 ACCEPT/PERMIT, REJECT/DENY, AND DISCARD

Different implementations use different terminology for the same actions. However, the behaviour is fundamentally the same. Accept or permit, if applied to a routing policy, causes prefixes matching the criteria to be installed into the routing table, redistributed to another routing protocol, or announced to a neighbour.