Policy Terminology

A policy is a mechanism in the JUNOS policy framework that allows you to configure criteria against which something can be compared and an action that is performed if the criteria are met.

All policies provide two points at which you can control routing information or packets through the router (see Figure 8.4). These control points allow you to control routing information before and after it is placed in the routing table, data packets before and after a forwarding table lookup, and local packets before and after they are received by the Routing Engine. (Figure 8.4 appears to depict only one control point, but because of the bidirectional flow of the local packets, two control points actually exist.)

Figure 8.4. Policy Control Points

Because there are two control points, you can configure policies that control the routing information or data packets before and after their interaction with their respective tables and local packets before and after their interaction with the Routing Engine. Import routing policies control the routing information that is placed in the routing tables, while export routing policies control the routing information that is advertised from the routing tables. Input firewall filters control packets that are received on a router interface, while output firewall filters control packets that are transmitted from a router interface.

All policies consist of the following configurable components :

• Match conditionsCriteria against which a route or packets are compared. You can configure one or more criteria. If all criteria match, one or more actions are applied.

• ActionsWhat happens if all criteria match. You can configure one or more actions.

• TermsNamed structures in which match conditions and actions are defined. You can define one or more terms.

The policy framework software evaluates each incoming and outgoing route or packet against the match conditions in a term . If the criteria in the match conditions are met, the defined action is taken. In general, the policy framework software compares the route or packet against the match conditions in the first term in the policy, then goes on to the next term, and so on. Therefore, the order in which you arrange terms in a policy is relevant. However, the order of match conditions within a term is not relevant because a route or packet must match all match conditions in a term for an action to be taken.

If an incoming or outgoing route or packet arrives and an explicitly configured policy related to the route or to the interface upon which the packet arrives is not configured, the action specified by the default policy is taken. A default policy is a rule or a set of rules that determine whether the route is placed in or advertised from the routing table, or whether the packet is accepted into or transmitted from the router interface. All policies also have default actions in case a policy does not specify a match condition, a match occurs, but a policy does not specify an action, a match does not occur with a term in a policy and subsequent terms in the same policy exist, or a match does not occur by the end of a policy.

All policies share a two-step configuration process:

• Define the policyDefine the policy components, including criteria against which routes or packets are compared and actions that are performed if the criteria are met.

• Apply the policyApply the policy to whatever moves the routing information or packets through the router, for example, the routing protocol or router interface.

The JUNOS policy architecture is simple and straightforward. However, the actual implementation of each policy adds layers of complexity to the respective policies as well as power and flexibility to your router's capabilities. Configuring a policy has a major impact on the flow of routing information or packets within and through the router. For example, you can configure a routing policy that does not allow routes associated with a particular customer to be placed in the routing table. As a result of this routing policy, the customer routes are not used to forward data packets to various destinations, and the routes are not advertised by the routing protocol to neighbors. Before configuring a policy, determine what you want to accomplish with it and thoroughly understand how to achieve your goal using the various match conditions and actions. Also, make sure that you understand the default policies and actions for the policy you are configuring.