Which type of OSPF LSAs can be summarized only on ASBR?

Type 5 Link State Advertisements (LSAs), also known as External LSAs, are specifically designed for advertising routes that originate from outside the OSPF autonomous system (AS) into the OSPF network. These LSAs are used by Autonomous System Boundary Routers (ASBRs) to distribute external routes within the OSPF domain.

Summarization of Type 5 LSAs occurs exclusively at the ASBRs because these routers are responsible for injecting external routing information into the OSPF network. This is done to reduce the size of the routing tables and limit the resource usage in an OSPF network by condensing multiple external routes into a single summarized route.

Therefore, it is essential that only ASBRs handle the summarization of Type 5 LSAs, as they are the only routers that are aware of the external routes. This handling is vital for maintaining an optimal and efficient routing environment within the OSPF architecture. The other types of LSAs, such as Type 1 (router LSAs), Type 2 (network LSAs), and Type 3 (summary LSAs), are not exclusive to ASBRs for summarization capabilities and serve different roles within the OSPF operation.