LSA (Link State Advertisement) Types in OSPF

Every OSPF speaking router floods LSA (Link state advertisement) to advertise the state of it's link, if link is down LSA is generated, when links comes up LSA will be generated.

With the flip flop of the link LSA will be generated and the latest state of the link will be advertised.

OSPF stores it's inofrmation in LSDB (Link state Database) and fills it with LSAs (Link State Advertisement), there are several different types of LSAs.

     I.    LSA Type 1                  Router LSA

    II.    LSA Type 2                  Network LSA

  III.    LSA Type 3                  Summary LSA

  IV.    LSA Type 4                  Summary ASBR LSA

   V.    LSA Type 5                  Autonomous System External LSA

  VI.    LSA Type 6                  Multicast OSPF LSA

VII.    LSA Type 7                  NSSA LSA

VIII.    LSA Type 8                  External Attribute LSA for BGP

If you are studying one LSA type it would be comparetively different to digest the concept but if you study and corelate all of them together it would be very easily to understand the whole picture.

LSA 1

When any router comes up, or its interface goes down it advertises its link state; this advertisement is called LSA 1.

Router multicasts its information (all entries) to DR, medium is broadcast and the address is multicast.

Now this is DR responsibilities to tell every router that these new entries have emerged update it into your database.

Example: - Hello everybody (broadcast), Hello all seniors (medium broadcast address multicast)

LSA 2

After an (upcoming) router has reported its link state to DR, what DR multicasts to all other devices is LSA 2.

Originator is present in every packet; it reads like, this packet is generated by [router-id]

LSA 3 & 4

They are also called summary LSA; anything that is originated by ABR is called summary LSA or LSA 3 and 4.

There is difference between LSA 3 and 4

Difference between LSA 3 & 4 is LSA 3 shows the network information whereas LSA 4 shows the location of ASBR (Tells IP address of ASBR as gateway).

We made areas to reduce the LSA traffic, if LSA are also sent across the areas then what is the benefit of areas? In-fact the LSA within an area are fully detailed LSA and that which are sent across the area are just summary LSA.

LSA 5

Also known as External LSA, it is anything originated by ASBR, from one area to another area only summary is sent not all the information is sent.

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