Autonomous System Boundary Routers 459Router placementand participationWhen you populate an area with OSPF routers, consider the followingguidelines:n Because OSPF uses a CPU-intensive algorithm, keep the maximumnumber of routers participating in OSPF exchanges in any given areato around 50. This number decreases the likelihood of performanceproblems that may be associated with router recalculation. If the link isof high quality and the number of routes is minimal, you can increasethe number of area routers.n Keep the maximum number of neighbors for any one router to around60. Each time that a topological change occurs, a router exchangesinformation only with those neighbors with which it has formed anadjacency. On a multiaccess network, this neighbor count is only ofconcern to the designated and backup-designated router of an areabecause, on a multiaccess network, area routers do not exchangelink-state information with each other. Instead, they exchangelink-state information with only the designated and backupdesignated routers.AutonomousSystem BoundaryRoutersAutonomous system boundary routers (ASBRs) are the links between theOSPF autonomous system and the outside network. They exchange theirautonomous system topology data with boundary routers in otherautonomous systems.ASBRs can import external link advertisements that contain informationabout external networks from other protocols like RIP and redistributethem as LSAs to the OSPF network. In this way, ASBRs flood informationabout external networks to routers within the OSPF network.Every router inside an autonomous system knows how to reach theboundary routers for its autonomous system.In Figure 54, two ASBRs control traffic between two autonomoussystems.Configuring an ASBR A router becomes an ASBR as a by-product of other settings. A routerbecomes an ASBR if any operational in-band IP interface on the router:n Has both OSPF and RIP disabled on that interface. Or,n Has RIP configured as learn, advertise, or enabled on that interface.
