Which term describes LSA types that are only advertised by Area Border Routers (ABRs)?

The correct choice indicates that Type 3 LSAs (Link-State Advertisements) are specifically advertised by Area Border Routers (ABRs) in the context of OSPF (Open Shortest Path First) routing protocols. Type 3 LSAs are used to summarize routes between different OSPF areas.

When OSPF operates in a multi-area environment, the ABRs facilitate communication between these areas by summarizing the routing information from one area and advertising it into another. This helps reduce the amount of routing information that must be exchanged and processed, enhancing the overall efficiency of the network.

In contrast, Type 1 LSAs are generated by routers and represent the router's own interfaces and links within a single area. Type 2 LSAs represent multi-access networks and are typically generated by the designated router of a broadcast network. Type 4 LSAs are sent by ABRs to advertise ASBR (Autonomous System Boundary Router) information, helping inter-area routing by indicating how to reach external networks.

Thus, the definition tied to Type 3 LSA – being the type specifically associated with ABRs for the purpose of summarizing inter-area routes – underscores why this term accurately describes LSAs that ABRs advertise.