A Type 5 LSA, or AS External LSA, is generated by an OSPF autonomous system boundary router (ASBR) to advertise routes to networks outside the OSPF autonomous system. A common example is a default route to the internet. section 18.4
Remember these exam-relevant points:
- ASBRs generate Type 5 LSAs. section 18.5
- The LSA is distributed through the OSPF link-state database (LSDB). section 18.4
- Use show ip ospf database to view Type-5 AS External Link States. section 18.4
Default route to the internet
In an OSPF network connected to the internet, R1 acts as the ASBR and advertises a default route with default-information originate. The LSDB on the routers then shows R1 advertising a Type 5 LSA for 0.0.0.0. section 18.4
Figure 18.14 An OSPF AS and the contents of its LSDB. Each router advertises a type 1 (Router) LSA. R5, as the DR of the R2–R3–R4–R5 segment, advertises a type 2 (Network) LSA. R1, as an ASBR, advertises a type 5 LSA.
Use the topology and LSDB diagram to connect the ASBR, the external connection, and the resulting Type 5 LSA. section 18.4
