Which two OSPF LSA types can be used to advertise routes between areas?

Correctly identifying the LSA types used to advertise routes between OSPF areas involves an understanding of how OSPF (Open Shortest Path First) operates across multi-area networks. OSPF uses Link State Advertisements (LSAs) to exchange routing information, with different types serving specific purposes.

External type LSAs, which are also known as Type 5 LSAs, are employed when routes from outside the OSPF routing domain need to be advertised into OSPF. These LSAs allow OSPF routers to learn about external routes, typically originating from other routing protocols, and thus support inter-area routing when external networks are involved.

Summary type LSAs, identified as Type 3 LSAs, are used to summarize routes between OSPF areas. They're generated by Area Border Routers (ABRs) and help to minimize the amount of routing information that needs to be exchanged between areas. By summarizing routes, OSPF can manage larger networks more efficiently, reducing the routing table size in non-backbone areas and enhancing scalability.

Both summary and external LSAs play vital roles in the functioning of OSPF in multi-area environments, facilitating the exchange of routing information between areas effectively.