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9.8 | ![]() |
BGP Route Filtering and Policy Routing | |
9.8.2 | ![]() |
Using filters to implement routing policy |
Two distinct steps are involved in manipulating a route or a set of
routes as follows:
The identification process typically relies on a route map. Prefixes can be selected by their destination network number, the AS from which the prefix originated, the AS_Path, or another specific attribute value. Prefixes are identified using the match statement from the route map. After a route map matches a given prefix, the actions specified by the route map will be executed, and processing will be considered complete. In other words, when a prefix matches, it will not be passed through any remaining clauses in the route map.What actions can the route map take after it has identified a match? The simplest actions are either to permit the route to pass through or to filter it out by denying it. Actions that are more complex adjust the attributes of a prefix to influence the routing process in some way. Notice that the route map can match a prefix based on several criteria, such as network number or AS_Path information. Also, notice that once a route matches there are no further comparisons. The order in which the matches are configured in the route map is important. If a route map clause that permits all routes is put at the beginning of the list, it overrides all the other policies configured.
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