LSA Types in OSPF

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LSA Types in OSPF

OSPF relies on several types of Link State Advertisements (LSAs) to communicate link state information between neighbours. A brief review of the most applicable LSA types:

LSA Types:

• Type 1 - Represents a router
• Type 2 – Network LSA (who is my DR)
• Type 3 - A network link summary all ABR (internal route)
• Type 4 - Represents an ASBR (who is my ASBR)
• Type 5 - A route external to the OSPF domain
• Type 6 – Group membership Multicasting
• Type 7 - Used in stub areas in place of a type 5 LSA

LSA types 1 and 2 are found in all areas, and are never flooded outside of an area. Whether the other types of LSAs are advertised within an area depends on the area type, and there are many:

             Note: The concept of Areas is to compress the LSA’s. So as to reduce the Traffic in Network.

Let's begin by examining a standard area. Note that the backbone area is essentially a standard area which has been designated as the central point to which all other areas connect, so a discussion of standard area behaviour largely applies to the backbone area as well.

LSA – 1:

Ø  Contains Link Information

Ø  Originated by all routers

Ø  Flooded within an area (it will not share with other area routers)

Ø  Learn as ‘O’ route

LSA – 2:

Ø  Contains Neighbour information

Ø  Originated by DR and BDR

Ø  Flooded within an area

Ø  Learn as ‘O’ route

LSA – 3:

Ø  Contains the link information

Ø  Originated by ABR

Ø  Flooded to adjacent area

Ø  Learn as ‘OIA’ route

LSA – 4:

Ø  Contains information about ASBR (Location of ASBR)

Ø  Originated by ABR of that particular area

Ø  Flooded to whole OSPF domain

LSA – 5:

Ø  Contains external ink information

Ø  Originated by ASBR

Ø  Flooded into whole OSPF domain

Ø  Learn as OE1 & OE2 routes

LSA – 6: ----

LSA – 7:

Ø  Contains external link information

Ø  Originated by NSSA ASBR

Ø  Floods to an NSSA area only

Ø  Learn as ON1 and ON2