7.5.2 Basic Route Filtering

Filtering of routing information allows control of paths used by a router to forward packets it receives. A router should be selective in which sources of routing information it listens to and what routes it believes. Therefore, a router MUST provide the ability to specify:

• On which logical interfaces routing information will be accepted and which routes will be accepted from each logical interface.

• Whether all routes or only a default route is advertised on a logical interface.

Some routing protocols do not recognize logical interfaces as a source of routing information. In such cases the router MUST provide the ability to specify

• from which other routers routing information will be accepted.

For example, assume a router connecting one or more leaf networks to the main portion or backbone of a larger network. Since each of the leaf networks has only one path in and out, the router can simply send a default route to them. It advertises the leaf networks to the main network.