As mentioned, route filtering might be required when redistributing routes. Route filtering prevents the advertisement or acceptance of certain routes through the routing domain. Filtering can be configured as follows:

■ On a routing domain boundary where redistribution occurs

■ Within the routing domain to isolate some parts of the network from other parts

■ To limit routing traffic from untrusted external domains Filtering is used with route redistribution, primarily to prevent suboptimal routing and routing loops that might occur when routes are redistributed at multiple redistribution points. Route filtering is also used to prevent routes about certain networks, such as a private IP address space, from being sent to or received from remote sites.