Cisco Certified Network Associate (CCNA) Practice Exam

What is the characteristic of Type 4 LSA in OSPF?

1. Describes inter-area routes

2. Indicates reachability to an ASBR

3. Contains information about external routes

4. Defines the network topology

The correct answer is: Indicates reachability to an ASBR

Type 4 Link-State Advertisements (LSAs) in OSPF (Open Shortest Path First) serve a crucial role in the routing process, specifically in relation to external routes. The primary function of Type 4 LSAs is to indicate the reachability to an Autonomous System Boundary Router (ASBR). This is vital in OSPF because ASBRs are responsible for redistributing routes from other routing protocols or networks into OSPF. By providing information that directs OSPF routers to where they can find the ASBR, Type 4 LSAs ensure that routers can locate and use external routes efficiently. This characteristic is integral for maintaining seamless communication within an OSPF network when integrating with other protocols or routing domains. Other types of LSAs serve different purposes, such as describing inter-area routes or containing details about external routes. However, the specific role of Type 4 LSA in notifying routers about the existence and reachability of an ASBR is what sets it apart within the OSPF domain.