IP Routing Information Filters

IP routing information filters let the router restrict the routes that it accepts from neighbors (incoming filters) and limit the advertised routes (outgoing filters). The router can use RIP, Open Shortest Path First (OSPF), or Exterior Gateway Protocol (EGP) to exchange routing information with other neighboring routers, as follows:

• RIP filters control the propagation of routing information and hide the existence of specific IP networks from other routers.
• OSPF filters control the propagation of routing information from non-OSPF domains (RIP, EGP, and so on) to the OSPF domain.
• EGP filters define the routes a router can share with other EGP peers.

IP Incoming Filters

IP incoming filters let the router restrict information about the routes it accepts from its neighbors. Filtered routes are not recorded in the local forwarding table and cannot be propagated to other routers or hosts. The filter includes the destination network and the source of the route. The source identifies the interface, interface group, or WAN connection from which the route information was received or the address of the router that provided the information.

You can apply IP incoming filters only to RIP and EGP routes. You cannot filter routes to directly connected networks. Incoming filters do the following:

• RIP incoming filters restrict the acceptance of routing information from other RIP routers.
• EGP incoming filters restrict the routes accepted from the EGP peers.

IP Outgoing Filters

IP outgoing filters restrict the propagation of route information from the router. You can also use them to control the flow of routes between the routing protocols. An outgoing filter specifies a route and a potential recipient of the information. The recipient is an outbound interface, an interface group, a WAN connection, or the IP address of another router.

You can apply IP outgoing filters to RIP, EGP, and OSPF routes. You cannot filter directly connected networks. The following outgoing IP filters are available:

• RIP outgoing filters restrict the advertising of routing information and hide the existence of specific IP networks from other routers.
• OSPF external route filters define the routes learned from RIP, EGP, or static routes that are propagated into the OSPF domain.
• EGP outgoing filters restrict the routes that are propagated to the EGP peers.