Route Filtering

Route Filtering — Purpose
Route filtering decides whether to permit or deny certain routing
updates from being accepted into, or distributed out of, the routeselection
process:
 Allow, or reject, routes from neighbor routers into, or from, the
route-selection process
 After determining the route-selection process, permit some routes
to be sent, or deny some routes from being sent, to neighbor
routers
Route filtering is a method of manipulating the entries in the router’s forwarding table. The denial of the import and
export of certain routes manipulates the way that traffic flows across a network.
Route filtering is not the filtering of packets and should not be confused with traffic filtering. Route filtering strictly
pertains to the manipulation of the routing entries for a particular router or network.

As shown in the figure above, the RIB consists of all routes to all destinations learned by all routing protocols that are

running on a particular router. Using a combination of metrics and preferences, the router selects the best route to

each destination, and populates the FIB with the selected best routes.

The use of route filters manipulates the information that populates the RIB and thus affects the information that is

placed in the FIB, which in turn affects traffic flow through the network.

The use of route filters can influence the content and size of the routing table, as well as the information that is

advertised to the router’s neighbors.