Link State Advertisements (LSAs) are the building blocks of OSPF. You need to know about OSPF LSA because there are many OSPF area types and certain area types block certain LSA type from entering the area. There are 11 types of OSPF LSAs, these are the 5 most common OSPF LSAs:

  • Router LSA (Type 1)

  • The most common type of LSA. An advertisement about one network. Like hey, this is network and here’s the cost to reach it.

  • Network LSA (DR Generated) (Type 2)

  • Only advertised by DR. Advertise all routers that are attached to the same segment (same switch/LAN) including itself. This LSA is sent to all of the routers in the area (not only to the routers in the same segment as the DR).

  • Summary LSA (ABR Summary route) (Type 3)

  • Advertisement about networks from another area. This route is marked as an IA (Inter-Area) route in the routing table.

  • Summary LSA (ASBR Location) (Type 4)

  • Advertise the ip address of the ASBR in the network. This might be needed to know the next-hop ip address for external routes.

  • External LSA (ASBR Summary route) (Type 5)

  • Advertised routes coming from an ASBR.

  • NSSA External LSA (Type 7)

  • Generated by the ASBR in an NSSA (Not-So-Stubby Area). This LSA is converted back to type 5 once it reach the backbone area