Understanding OSPF Routing Policy
Each routing policy is identified by a policy name. The name can contain letters, numbers, and hyphens (-) and can be up to 255 characters long. To include spaces in the name, enclose the entire name in double quotation marks. Each routing policy name must be unique within a configuration. Once a policy is created and named, it must be applied before it is active.
In the import statement, you list the name of the routing policy used to filter OSPF external routes from being installed into the routing tables of OSPF neighbors. You can filter the routes, but not link-state address (LSA) flooding. An external route is a route that is outside the OSPF Autonomous System (AS). The import policy does not impact the OSPF database. This means that the import policy has no impact on the link-state advertisements.
In the export statement, you list the name of the routing policy to be evaluated when routes are being exported from the routing table into OSPF.
By default, if a routing device has multiple OSPF areas, learned routes from other areas are automatically installed into area 0 of the routing table.
To specify more than one policy and create a policy chain, you list the policies using a space as a separator. If multiple policies are specified, the policies are evaluated in the order in which they are specified. As soon as an accept or reject action is executed, the policy chain evaluation ends.
This topic describes the following information:
Routing Policy Terms
Routing policies are made up of one or more terms. A term is a named structure in which match conditions and actions are defined. You can define one or more terms. The name can contain letters, numbers, and hyphens ( - ) and can be up to 255 characters long. To include spaces in the name, enclose the entire name in double quotation marks.
Each term contains a set of match conditions and a set of actions:
Match conditions are criteria that a route must match before the actions can be applied. If a route matches all criteria, one or more actions are applied to the route.
Actions specify whether to accept or reject the route, control how a series of policies are evaluated, and manipulate the characteristics associated with a route.
Routing Policy Match Conditions
A match condition defines the criteria that a route must match for an action to take place. You can define one or more match conditions for each term. If a route matches all of the match conditions for a particular term, the actions defined for that term are processed.
Each term can include two statements, from and to, that define the match conditions:
In the from statement, you define the criteria that an incoming route must match. You can specify one or more match conditions. If you specify more than one, they all must match the route for a match to occur.
The from statement is optional. If you omit the from and the to statements, all routes are considered to match.
In export policies, omitting the from statement from a routing policy term might lead to unexpected results.
In the to statement, you define the criteria that an outgoing route must match. You can specify one or more match conditions. If you specify more than one, they all must match the route for a match to occur.
The order of the match conditions in a term is not important because a route must match all match conditions in a term for an action to be taken.
For a complete list of match conditions, see Configuring Match Conditions in Routing Policy Terms.
Routing Policy Actions
An action defines what the routing device does with the route when the route matches all the match conditions in the from and to statements for a particular term. If a term does not have from and to statements, all routes are considered to match and the actions apply to all routes.
Each term can have one or more of the following types of actions. The actions are configured under the then statement.
Flow control actions, which affect whether to accept or reject the route and whether to evaluate the next term or routing policy.
Actions that manipulate route characteristics.
Trace action, which logs route matches.
The then statement is optional. If you omit it, one of the following occurs:
The next term in the routing policy, if one exists, is evaluated.
If the routing policy has no more terms, the next routing policy, if one exists, is evaluated.
If there are no more terms or routing policies, the accept or reject action specified by the default policy is executed.
For a complete list of routing policy actions, see Configuring Actions in Routing Policy Terms.