Advanced IP Network Design (CCIE Professional Development)

External Route Injection

External routes—routes from other autonomous systems or protocols—are injected into OSPF by autonomous system boundary routers (ASBRs). External routes are flooded throughout the OSPF autonomous system (throughout all areas) without change. (This means no summarization.) External routes within OSPF also have a Forward Address field, which allows an OSPF router to act as a route server.

In Figure A-9, Router B is an ASBR for the OSPF cloud and is also learning routes from Router A and Router C through the Border Gateway Protocol (BGP). Router D is not learning these BGP routes, but it is advertising an internal OSPF link to the Ethernet. When Router B advertises these routes it has learned from BGP, it will put the Ethernet ...