OSPF Routing Policies 403OSPF RoutingPoliciesRouting policies are rules that define criteria to control the flow of routesto and from the routing table. Your system supports two types of OSPFrouting policies: import policies that dictate which routes are added tothe routing table and export polices that dictate which routes areadvertised to other routers. You can use routing policies to:n Increase security — For security reasons, you may not want therouter to advertise certain routes. For example, Organization A mayhave defined one of its ASBRs with a direct connection toOrganization B that they use for direct communication. For security orperformance reasons, A may not want to give other groups access tothat connection. To prevent this direct connection from being knownto other organizations, A can define an export policy that prohibits itsASBR from advertising the direct connection that it uses tocommunicate with B.n Conserve routing table space — The selective nature of routingpolicies can minimize routing table sizes and increase network stability.For example, you may want to limit the number of hosts andgateways from which routing information is accepted, in which caseyou can define an import policy to selectively rule out, or reject,unnecessary routing table entries.n Isolate suspect networks — Misconfigured hosts can sometimessend inappropriate routing information, which can compromisenetwork integrity. In such a case, you can define an import policy onan ASBR that rejects all routes from the suspect network.n Adjust route cost — Both import and export policies let you changethe cost that is associated with routes without physically changing thecost of an interface. For example, router A may advertise a route withone cost, but router B may use an import policy to write the sameroute to its routing table with a different, or adjusted, cost. Similarly,router A may have a route in its routing table with one cost butchoose to advertise the route to other routers with a different cost.
