What is OSPF LSA types?
Answer
OSPF uses different types of LSAs (Link State Advertisements) to distribute topology information. Type 1 (Router LSA): generated by every router, describes its directly connected links and states — flooded only within an area. Type 2 (Network LSA): generated by the DR (Designated Router) on broadcast/multi-access networks, lists all routers on the segment — flooded within an area. Type 3 (Summary LSA): generated by ABRs (Area Border Routers), advertises inter-area routes — flooded between areas. Type 4 (ASBR Summary LSA): generated by ABRs, tells other areas how to reach the ASBR. Type 5 (AS External LSA): generated by ASBRs, advertises external routes (from other routing protocols) — flooded through the entire OSPF domain (except stub areas). Type 7 (NSSA External LSA): used in NSSAs instead of Type 5, converted to Type 5 by ABR. Stub areas block Type 4 and 5 LSAs, reducing the LSDB size. Understanding LSA types is essential for OSPF troubleshooting and design.