9.2
Route Filtering and Manipulation Process (Policies)
9.2.2
Route filtering and manipulation process actions
There are two distinct steps to manipulating a route or a set of routes:
  1. Identify the prefixes you would like the policies to apply to
  2. Implementing the policies (which can be filtering prefixes out altogether or manipulating the attributes of a prefix to influence the routing decision).

Identifying Routes

Prefixes can be selected by their destination network number, the AS from which the prefix originated, a list of ASs the prefix has passed through, a specific attribute value, and so on. Prefixes are identified using match commands within the various clauses of a route map; after a given prefix has matched, the actions specified within that clause of the route map will be executed, and processing will be considered complete. In other words, once a prefix matches, it won't be passed through any remaining clauses in the route map for consideration.

Permitting or Denying the Routes

Once a prefix has been identified it can be permitted or denied. The criteria for permitting or denying routes depend on the policies an AS is setting; if the route is denied, it is discarded.

Manipulating Attributes

The attributes of a prefix can also be changed to affect the BGP best path decision process. The figure illustrates how multiple route map clauses can be applied on a set of routes to find a match. Note that each clause can have a match on one or more criteria: for example, a prefix could be checked based on its network number (address) as well as its AS_path information..

Also note that after a route matches, it is not compared to any more clauses in the route map-so the order in which the matches are configured is important. A route map clause that permits all routes, for example, if put at the beginning of the list, will override all the other policies configured.