Routing with OSPF – Part 4 (OSPF LSA Types) ⋆ IpCisco

In OSPF Link-State information, in other words,Routing information is exchanged between routers. This exchange is done with LSAs (Link State Advertisements).

There are eleven different LSA types and each of them has a special purpose. Some of the LSA types are the main LSA types that are used in OSPF operations often. Let’s see all the LSA types:

• Router LSA (Type 1)
• Network LSA (Type 2)
• ABR Summary LSA (Type 3)
• ASBR Summary LSA (Type 4)
• ASBR Externel LSA (Type 5)
• Group Summary (Type 6) :
• NSSA External LSA (Type 7)
• External Attributes LSA (Type 8)
• Opaque LSAs (Type 9, 10, 11)

Now, let’s talk about these OSPF LSA types detailly. Here, it is better to explain these LSA types with different area types and their LSA Adversisement requirements. So, the below diagram will be very useful for you.

OSPF Router LSA (Type 1)

Router LSA (Type 1) is used for only Internal Routers in a Standard Area. Router LSA (Type 1) list router neighbors and their costs. This LSA type can not pass throught the ABR (Area Border Router) and can not go to the other areas.

OSPF Network LSA (Type 2)

Network LSA (Type 2) is advertised only from DR and contains all the routers that is its adjacent on the segment. Network LSA (Type 2) also can not pass throught the ABR and can not go to the other areas.

OSPF ABR Summary LSA (Type 3)

ABR Summary LSA (Type 3) is generated by ABR (Area Border Router) to advertise one Area’s networks to other Areas. ABR Summary LSA (Type 3) includes all prefixes available in the Area.

OSPF ASBR Summary LSA (Type 4)

ASBR Summary LSA (Type 4) is generated by ABR (Area Border Router) to inform its areas about how to reach the ASBR (Autonomous System Border Router). ASBR Summary LSA (Type 4) includes ASBR’s Router ID.