Cisco Certified Network Associate (CCNA) Practice Exam

What type of LSA is used to propagate external routes, specifically from another routing protocol into OSPF?

1. Type 1 - Router LSA

2. Type 2 - Network LSA

3. Type 5 - External LSA

4. Type 4 - ASBR Summary LSA

The correct answer is: Type 5 - External LSA

The correct answer is indeed that the type of LSA used to propagate external routes, specifically from another routing protocol into OSPF, is the External LSA, known as Type 5. External LSAs are essential for OSPF to include routes that originate outside of the OSPF domain, allowing OSPF to advertise routes learned from other routing protocols, such as BGP or EIGRP. When OSPF receives a route from another routing protocol, it uses Type 5 LSAs to distribute this external routing information throughout the OSPF network. This ensures that all OSPF routers are aware of these external routes, enabling efficient routing decisions. Other types of LSAs serve different purposes in the OSPF protocol. For example, Router LSAs (Type 1) describe the state of the router and its links, while Network LSAs (Type 2) describe multi-access networks and their connected routers. The ASBR Summary LSA (Type 4) provides a summary of the external routes to other OSPF areas but does not directly carry the external route information itself. Thus, the Type 5 External LSA is specifically designated for the announcement of external routes into the OSPF network.