Link State Advertisements OSPF

The fundamental building blocks of OSPF are the link state advertisements that are sent from every router to advertise links and their states. Given the complexity and scalability of OSPF, different LSA types are used to keep the OSPF database updated. Out of the various LSAs, the first five are most relevant to the limited OSPF discussion covered in this chapter and are discussed below:

• Type 1 – Router LSA – Each router in the area sends this LSA to announce its presence and list the links to other routers and networks along with metrics to them. These LSAs do not cross the boundary of an area.
• Type 2 – Network LSA – The DR in a multi-access network sends out this LSA. It contains a list of routers that are present in the network segment. These LSAs also do not cross the boundary of an area.
• Type 3 – Summary LSA – The ABR takes the information learned in one area (and optionally summarizes this information) and sends it out to another area it is attached to. This information is contained in LSA type 3 and is responsible for propagation of Inter-area routes.
• Type 4 – ASBR Summary LSA – ASBRs originate external routes (redistributed routes) and send them throughout the network. While the external routes are listed in type 5 LSA, the details of the ASBR themselves in listed in type 4 LSAs. This LSA is originated by the ABR of the area where the ASBR resides.
• Type 5 – External LSA – This LSA lists routes redistributed into OSPF from another OSPF process or another routing protocol. This LSA is originated by the ASBR and propagates across the OSPF AS.