As the topology of a network grows more complex, the need for more complex route filtering arises. Therefore, a router SHOULD provide the ability to specify independently for each routing protocol:
In many situations it is desirable to assign a reliability ordering to routing information received from another router instead of the simple believe or don't believe choice listed in the first bullet above. A router MAY provide the ability to specify:
If a router supports assignment of preferences, the router MUST NOT propagate any routes it does not prefer as first party information. If the routing protocol being used to propagate the routes does not support distinguishing between first and third party information, the router MUST NOT propagate any routes it does not prefer.
For example, assume a router receives a route to network C from router R and a route to the same network from router S. If router R is considered more reliable than router S traffic destined for network C will be forwarded to router R regardless of the route received from router S.
Routing information for routes which the router does not use (router S in the above example) MUST NOT be passed to any other router.