What are the types of OSPF LSAs ?

Router Link State Advertisments Type1 are generated by every router for each link that belongs to an area. They are flooded only inside of area to which they belong. Link ID of this LSA is the Router ID of the router that generated it.

Generated by Designated Router (DR) for multiaccess networks and describe the routers that are connected to that segment. They are sent inside the area to which the network segment belong. The Link ID is the interface ip address of the Designated Router which describe that particular segment.

Type3 LSAs are generated by Area Border Routers (ABRs). In type3 LSAs are advertised networks from an area to the rest of the areas in AS. Advertised networks in Type4 LSA can be summarized or not. The link-state id used by this LSA is the network number advertised.

Generated by ABRs, this type os LSA contain routes to ASBRs. Link id used is router ID of the ASBR described. Are not flooded in stub areas.

Autonomous system external LSAs are generated by ASBRs and contain routes to networks that are external to current AS. Link-state ID is network number advertised in LSA. Type5 LSAs are not flooded inside any stub areas.

This type of LSA is used for multicast applications.

Type7 LSA allow injection of external routes throug Not-so-Stubby-Areas (NSSA). Generally external routes are advertised by type5 LSA  but they are not allowed inside any stub area. That’s why Type7 LSA is used, to trick OSPF. Type7 LSA is generated by NSSA ASBR and is translated into type5 LSA as it leaves the area by NSSA ABR, which is then propagated throughout the network as type5 LSA.

Type1 – Router LSA: The Router LSA is generated by each router for each area it is

located. In the link-state ID you will find the originating router’s ID. I’m showing you

the router LSAs only in area0.

• Type2 – Network LSA: Network LSAs are generated by the DR. The link-state ID will

be the router ID of the DR. In area1 you can see a DR which is sending a network

LSA.

• Type3 – Summary LSA: The summary LSA is created by the ABR and flooded into

other areas. This way area2 will learn about the prefixes in area0 and1.

• Type4 – Summary ASBR LSA: Other routers need to know where to find the ASBR.

This is why the ABR will generate a summary ASBR LSA which will include the router

ID of the ASBR in the link-state ID field.

• Type5 – External LSA: also known as autonomous system external LSA: The

external LSAs are generated by the ASBR. There is a ASBR in area2 which is

generating these LSAs so our OSPF domain knows about the prefixes that are

redistributed from BGP.

• Type7 – External LSA: also known as not-so-stubby-area (NSSA) LSA: As you can

see area1 is a NSSA (not-so-stubby-area) which doesn’t allow external LSAs (type

5). To overcome this issue we are generating type7 LSAs so our OSPF domain

knows about the prefixes that are redistributed from RIP

Type1 - Represents a router

Type2 - Represents the designated router for a multiaccess link

Type3 - A network link summary (internal route)

Type4 - Represents an ASBR

Type5 - A route external to the OSPF domain

Type7 - Used in stub areas in place of a type5 LSA