A little OSPF LSA types cisco cool explanation

Type-1 LSA – It’s is the Router LSA; this is defined in order to send the link state information about the attributes of the directly connected routes, besides of that you have just ONE type-1 LSA per every router inside your area.

Type-2 LSA – It’s the Network LSA; this LSA is specifically defined for Broadcast and Non-Broadcast network types. This is managed by the DR of the segment and there is just ONE type-2 LSA per segment.

Type-3 LSA - This is the Summary-net LSA; this LSA is normally in designs where you have multiple areas, so the ABR is in charged to convert the intra-area LSA types to this type of LSA in order to differentiate the intra-area routes from the inter-are routes. There is one type-3 LSA per route.

Type-4 LSA - This is the ASBR Summary LSA this basically identifies the ASBR of a specific area (note that inside the area where the ASBR is this LSA will not exist, instead the type-1 LSA will be used in order to announce the ASBR of the area). Who is the ASBR of the area? The type-4 LSA is there in order to answer this question, so this is injected by the ABR in order to announce the presence of the ASBR in a particular area.

LSA-type 5 – This is the ASBR External LSA; these basically are the external routes, one type-5 LSA per route.

LSA-type 7 - This is a special LSA used for special area types like NSSA and Totally-Stubby NSSA, this basically resides in a particular area and doesn't purely go out instead is translated by the ABR to type-5 LSA. This logic is very important because this is the foundation to learn how the NSSA and Totally Stubby NSSA special areas work.