Type 4 and type 5 LSAs are used collectively to identify an ASBR and advertise external networks into an OSPF routing domain.

A type 4 summary LSA is generated by an ABR only when an ASBR exists within an area. A type 4 LSA identifies the ASBR and provides a route to it. All traffic destined to an external autonomous system requires routing table knowledge of the ASBR that originated the external routes.

As shown in the figure, the ASBR sends a type 1 LSA, identifying itself as an ASBR. The LSA includes a special bit known as the external bit (e bit) that is used to identify the router as an ASBR. When ABR1 receives the type 1 LSA, it notices the e bit, it builds a type 4 LSA, and then floods the type 4 LSA to the backbone (area 0). Subsequent ABRs flood the type 4 LSA into other areas.

The link-state ID is set to the ASBR router ID.